diff --git a/.php-cs-fixer.php b/.php-cs-fixer.php
index 8e567fe0c369..62d1ac7f74fa 100644
--- a/.php-cs-fixer.php
+++ b/.php-cs-fixer.php
@@ -65,7 +65,7 @@
         // TODO later once 2.2 is more stable
         // 'array_syntax' => true,
         // 'list_syntax' => true,
-        'visibility_required' => ['elements' => ['property', 'method', /* TODO 'const' but need to review them all */]],
+        'visibility_required' => ['elements' => ['property', 'method', 'const']],
         'non_printable_character' => true,
         'combine_nested_dirname' => true,
         'random_api_migration' => true,
diff --git a/phpstan/baseline.neon b/phpstan/baseline.neon
index 747e898cec53..badf948db226 100644
--- a/phpstan/baseline.neon
+++ b/phpstan/baseline.neon
@@ -5495,6 +5495,14 @@ parameters:
 			count: 1
 			path: ../src/Composer/Repository/PlatformRepository.php
 
+		-
+			message: """
+				#^Fetching deprecated class constant PLATFORM_PACKAGE_REGEX of class Composer\\\\Repository\\\\PlatformRepository\\:
+				use PlatformRepository\\:\\:isPlatformPackage\\(string \\$name\\) instead$#
+			"""
+			count: 1
+			path: ../src/Composer/Repository/PlatformRepository.php
+
 		-
 			message: "#^Only booleans are allowed in &&, mixed given on the right side\\.$#"
 			count: 1
diff --git a/src/Composer/Command/BaseDependencyCommand.php b/src/Composer/Command/BaseDependencyCommand.php
index 5dfbd9fa1c39..9f5b4a7e1688 100644
--- a/src/Composer/Command/BaseDependencyCommand.php
+++ b/src/Composer/Command/BaseDependencyCommand.php
@@ -36,10 +36,10 @@
  */
 class BaseDependencyCommand extends BaseCommand
 {
-    const ARGUMENT_PACKAGE = 'package';
-    const ARGUMENT_CONSTRAINT = 'version';
-    const OPTION_RECURSIVE = 'recursive';
-    const OPTION_TREE = 'tree';
+    protected const ARGUMENT_PACKAGE = 'package';
+    protected const ARGUMENT_CONSTRAINT = 'version';
+    protected const OPTION_RECURSIVE = 'recursive';
+    protected const OPTION_TREE = 'tree';
 
     /** @var ?string[] */
     protected $colors;
diff --git a/src/Composer/Command/SelfUpdateCommand.php b/src/Composer/Command/SelfUpdateCommand.php
index af0b1007eb26..c0b688e585bd 100644
--- a/src/Composer/Command/SelfUpdateCommand.php
+++ b/src/Composer/Command/SelfUpdateCommand.php
@@ -36,8 +36,8 @@
  */
 class SelfUpdateCommand extends BaseCommand
 {
-    const HOMEPAGE = 'getcomposer.org';
-    const OLD_INSTALL_EXT = '-old.phar';
+    private const HOMEPAGE = 'getcomposer.org';
+    private const OLD_INSTALL_EXT = '-old.phar';
 
     /**
      * @return void
diff --git a/src/Composer/Command/StatusCommand.php b/src/Composer/Command/StatusCommand.php
index 8aec0e70dcdb..58b6663a719a 100644
--- a/src/Composer/Command/StatusCommand.php
+++ b/src/Composer/Command/StatusCommand.php
@@ -32,9 +32,9 @@
  */
 class StatusCommand extends BaseCommand
 {
-    const EXIT_CODE_ERRORS = 1;
-    const EXIT_CODE_UNPUSHED_CHANGES = 2;
-    const EXIT_CODE_VERSION_CHANGES = 4;
+    private const EXIT_CODE_ERRORS = 1;
+    private const EXIT_CODE_UNPUSHED_CHANGES = 2;
+    private const EXIT_CODE_VERSION_CHANGES = 4;
 
     /**
      * @return void
diff --git a/src/Composer/Composer.php b/src/Composer/Composer.php
index 8913397b6f43..a4b769ee2207 100644
--- a/src/Composer/Composer.php
+++ b/src/Composer/Composer.php
@@ -29,6 +29,8 @@ class Composer extends PartialComposer
     /*
      * Examples of the following constants in the various configurations they can be in
      *
+     * You are probably better off using Composer::getVersion() though as that will always return something usable
+     *
      * releases (phar):
      * const VERSION = '1.8.2';
      * const BRANCH_ALIAS_VERSION = '';
@@ -46,11 +48,13 @@ class Composer extends PartialComposer
      * const BRANCH_ALIAS_VERSION = '@package_branch_alias_version@';
      * const RELEASE_DATE = '@release_date@';
      * const SOURCE_VERSION = '1.8-dev+source';
+     *
+     * @see getVersion()
      */
-    const VERSION = '@package_version@';
-    const BRANCH_ALIAS_VERSION = '@package_branch_alias_version@';
-    const RELEASE_DATE = '@release_date@';
-    const SOURCE_VERSION = '2.3.999-dev+source';
+    public const VERSION = '@package_version@';
+    public const BRANCH_ALIAS_VERSION = '@package_branch_alias_version@';
+    public const RELEASE_DATE = '@release_date@';
+    public const SOURCE_VERSION = '2.3.999-dev+source';
 
     /**
      * Version number of the internal composer-runtime-api package
@@ -61,7 +65,7 @@ class Composer extends PartialComposer
      *
      * @var string
      */
-    const RUNTIME_API_VERSION = '2.2.2';
+    public const RUNTIME_API_VERSION = '2.2.2';
 
     /**
      * @return string
diff --git a/src/Composer/Config.php b/src/Composer/Config.php
index 57c54a09772e..b009af3820ec 100644
--- a/src/Composer/Config.php
+++ b/src/Composer/Config.php
@@ -24,11 +24,11 @@
  */
 class Config
 {
-    const SOURCE_DEFAULT = 'default';
-    const SOURCE_COMMAND = 'command';
-    const SOURCE_UNKNOWN = 'unknown';
+    public const SOURCE_DEFAULT = 'default';
+    public const SOURCE_COMMAND = 'command';
+    public const SOURCE_UNKNOWN = 'unknown';
 
-    const RELATIVE_PATHS = 1;
+    public const RELATIVE_PATHS = 1;
 
     /** @var array<string, mixed> */
     public static $defaultConfig = array(
diff --git a/src/Composer/DependencyResolver/Decisions.php b/src/Composer/DependencyResolver/Decisions.php
index f9e3490244c3..f06a25a8b74d 100644
--- a/src/Composer/DependencyResolver/Decisions.php
+++ b/src/Composer/DependencyResolver/Decisions.php
@@ -20,8 +20,8 @@
  */
 class Decisions implements \Iterator, \Countable
 {
-    const DECISION_LITERAL = 0;
-    const DECISION_REASON = 1;
+    public const DECISION_LITERAL = 0;
+    public const DECISION_REASON = 1;
 
     /** @var Pool */
     protected $pool;
diff --git a/src/Composer/DependencyResolver/Operation/InstallOperation.php b/src/Composer/DependencyResolver/Operation/InstallOperation.php
index 7f66d17e9c4b..da31a52cccc1 100644
--- a/src/Composer/DependencyResolver/Operation/InstallOperation.php
+++ b/src/Composer/DependencyResolver/Operation/InstallOperation.php
@@ -21,7 +21,7 @@
  */
 class InstallOperation extends SolverOperation implements OperationInterface
 {
-    const TYPE = 'install';
+    protected const TYPE = 'install';
 
     /**
      * @var PackageInterface
diff --git a/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php b/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php
index e1c21008b574..8626813e2571 100644
--- a/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php
+++ b/src/Composer/DependencyResolver/Operation/MarkAliasInstalledOperation.php
@@ -21,7 +21,7 @@
  */
 class MarkAliasInstalledOperation extends SolverOperation implements OperationInterface
 {
-    const TYPE = 'markAliasInstalled';
+    protected const TYPE = 'markAliasInstalled';
 
     /**
      * @var AliasPackage
diff --git a/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php b/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php
index 6cdd02837f1c..1e858ef1e65f 100644
--- a/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php
+++ b/src/Composer/DependencyResolver/Operation/MarkAliasUninstalledOperation.php
@@ -21,7 +21,7 @@
  */
 class MarkAliasUninstalledOperation extends SolverOperation implements OperationInterface
 {
-    const TYPE = 'markAliasUninstalled';
+    protected const TYPE = 'markAliasUninstalled';
 
     /**
      * @var AliasPackage
diff --git a/src/Composer/DependencyResolver/Operation/SolverOperation.php b/src/Composer/DependencyResolver/Operation/SolverOperation.php
index 5aa0957a75d0..094029af0e46 100644
--- a/src/Composer/DependencyResolver/Operation/SolverOperation.php
+++ b/src/Composer/DependencyResolver/Operation/SolverOperation.php
@@ -19,7 +19,10 @@
  */
 abstract class SolverOperation implements OperationInterface
 {
-    const TYPE = null;
+    /**
+     * @abstract must be redefined by extending classes
+     */
+    protected const TYPE = '';
 
     /**
      * Returns operation type.
diff --git a/src/Composer/DependencyResolver/Operation/UninstallOperation.php b/src/Composer/DependencyResolver/Operation/UninstallOperation.php
index 7b4ff8ddb3f0..878691508b51 100644
--- a/src/Composer/DependencyResolver/Operation/UninstallOperation.php
+++ b/src/Composer/DependencyResolver/Operation/UninstallOperation.php
@@ -21,7 +21,7 @@
  */
 class UninstallOperation extends SolverOperation implements OperationInterface
 {
-    const TYPE = 'uninstall';
+    protected const TYPE = 'uninstall';
 
     /**
      * @var PackageInterface
diff --git a/src/Composer/DependencyResolver/Operation/UpdateOperation.php b/src/Composer/DependencyResolver/Operation/UpdateOperation.php
index e674376f5fc3..728bb15116fc 100644
--- a/src/Composer/DependencyResolver/Operation/UpdateOperation.php
+++ b/src/Composer/DependencyResolver/Operation/UpdateOperation.php
@@ -22,7 +22,7 @@
  */
 class UpdateOperation extends SolverOperation implements OperationInterface
 {
-    const TYPE = 'update';
+    protected const TYPE = 'update';
 
     /**
      * @var PackageInterface
diff --git a/src/Composer/DependencyResolver/Request.php b/src/Composer/DependencyResolver/Request.php
index dde4972f5b85..d7cde079377a 100644
--- a/src/Composer/DependencyResolver/Request.php
+++ b/src/Composer/DependencyResolver/Request.php
@@ -26,19 +26,19 @@ class Request
     /**
      * Identifies a partial update for listed packages only, all dependencies will remain at locked versions
      */
-    const UPDATE_ONLY_LISTED = 0;
+    public const UPDATE_ONLY_LISTED = 0;
 
     /**
      * Identifies a partial update for listed packages and recursively all their dependencies, however dependencies
      * also directly required by the root composer.json and their dependencies will remain at the locked version.
      */
-    const UPDATE_LISTED_WITH_TRANSITIVE_DEPS_NO_ROOT_REQUIRE = 1;
+    public const UPDATE_LISTED_WITH_TRANSITIVE_DEPS_NO_ROOT_REQUIRE = 1;
 
     /**
      * Identifies a partial update for listed packages and recursively all their dependencies, even dependencies
      * also directly required by the root composer.json will be updated.
      */
-    const UPDATE_LISTED_WITH_TRANSITIVE_DEPS = 2;
+    public const UPDATE_LISTED_WITH_TRANSITIVE_DEPS = 2;
 
     /** @var ?LockArrayRepository */
     protected $lockedRepository;
diff --git a/src/Composer/DependencyResolver/Rule.php b/src/Composer/DependencyResolver/Rule.php
index bcf32fb03fd0..55e75e624768 100644
--- a/src/Composer/DependencyResolver/Rule.php
+++ b/src/Composer/DependencyResolver/Rule.php
@@ -29,19 +29,19 @@
 abstract class Rule
 {
     // reason constants and // their reason data contents
-    const RULE_ROOT_REQUIRE = 2; // array{packageName: string, constraint: ConstraintInterface}
-    const RULE_FIXED = 3; // array{package: BasePackage}
-    const RULE_PACKAGE_CONFLICT = 6; // Link
-    const RULE_PACKAGE_REQUIRES = 7; // Link
-    const RULE_PACKAGE_SAME_NAME = 10; // string (package name)
-    const RULE_LEARNED = 12; // int (rule id)
-    const RULE_PACKAGE_ALIAS = 13; // BasePackage
-    const RULE_PACKAGE_INVERSE_ALIAS = 14; // BasePackage
+    public const RULE_ROOT_REQUIRE = 2; // array{packageName: string, constraint: ConstraintInterface}
+    public const RULE_FIXED = 3; // array{package: BasePackage}
+    public const RULE_PACKAGE_CONFLICT = 6; // Link
+    public const RULE_PACKAGE_REQUIRES = 7; // Link
+    public const RULE_PACKAGE_SAME_NAME = 10; // string (package name)
+    public const RULE_LEARNED = 12; // int (rule id)
+    public const RULE_PACKAGE_ALIAS = 13; // BasePackage
+    public const RULE_PACKAGE_INVERSE_ALIAS = 14; // BasePackage
 
     // bitfield defs
-    const BITFIELD_TYPE = 0;
-    const BITFIELD_REASON = 8;
-    const BITFIELD_DISABLED = 16;
+    private const BITFIELD_TYPE = 0;
+    private const BITFIELD_REASON = 8;
+    private const BITFIELD_DISABLED = 16;
 
     /** @var int */
     protected $bitfield;
diff --git a/src/Composer/DependencyResolver/RuleSet.php b/src/Composer/DependencyResolver/RuleSet.php
index d430b46a9339..9d856c4b68f7 100644
--- a/src/Composer/DependencyResolver/RuleSet.php
+++ b/src/Composer/DependencyResolver/RuleSet.php
@@ -21,9 +21,9 @@
 class RuleSet implements \IteratorAggregate, \Countable
 {
     // highest priority => lowest number
-    const TYPE_PACKAGE = 0;
-    const TYPE_REQUEST = 1;
-    const TYPE_LEARNED = 4;
+    public const TYPE_PACKAGE = 0;
+    public const TYPE_REQUEST = 1;
+    public const TYPE_LEARNED = 4;
 
     /**
      * READ-ONLY: Lookup table for rule id to rule object
diff --git a/src/Composer/DependencyResolver/Solver.php b/src/Composer/DependencyResolver/Solver.php
index e90c3b67e731..0a07203983af 100644
--- a/src/Composer/DependencyResolver/Solver.php
+++ b/src/Composer/DependencyResolver/Solver.php
@@ -23,8 +23,8 @@
  */
 class Solver
 {
-    const BRANCH_LITERALS = 0;
-    const BRANCH_LEVEL = 1;
+    private const BRANCH_LITERALS = 0;
+    private const BRANCH_LEVEL = 1;
 
     /** @var PolicyInterface */
     protected $policy;
diff --git a/src/Composer/DependencyResolver/SolverProblemsException.php b/src/Composer/DependencyResolver/SolverProblemsException.php
index 936c73d744a1..caec38a34ae6 100644
--- a/src/Composer/DependencyResolver/SolverProblemsException.php
+++ b/src/Composer/DependencyResolver/SolverProblemsException.php
@@ -22,7 +22,7 @@
  */
 class SolverProblemsException extends \RuntimeException
 {
-    const ERROR_DEPENDENCY_RESOLUTION_FAILED = 2;
+    public const ERROR_DEPENDENCY_RESOLUTION_FAILED = 2;
 
     /** @var Problem[] */
     protected $problems;
diff --git a/src/Composer/Downloader/PathDownloader.php b/src/Composer/Downloader/PathDownloader.php
index 48a01ef613be..8cb7865e8ed7 100644
--- a/src/Composer/Downloader/PathDownloader.php
+++ b/src/Composer/Downloader/PathDownloader.php
@@ -33,8 +33,8 @@
  */
 class PathDownloader extends FileDownloader implements VcsCapableDownloaderInterface
 {
-    const STRATEGY_SYMLINK = 10;
-    const STRATEGY_MIRROR = 20;
+    private const STRATEGY_SYMLINK = 10;
+    private const STRATEGY_MIRROR = 20;
 
     /**
      * @inheritDoc
diff --git a/src/Composer/IO/IOInterface.php b/src/Composer/IO/IOInterface.php
index 66fe735db3a5..ba987cc8a390 100644
--- a/src/Composer/IO/IOInterface.php
+++ b/src/Composer/IO/IOInterface.php
@@ -22,11 +22,11 @@
  */
 interface IOInterface extends LoggerInterface
 {
-    const QUIET = 1;
-    const NORMAL = 2;
-    const VERBOSE = 4;
-    const VERY_VERBOSE = 8;
-    const DEBUG = 16;
+    public const QUIET = 1;
+    public const NORMAL = 2;
+    public const VERBOSE = 4;
+    public const VERY_VERBOSE = 8;
+    public const DEBUG = 16;
 
     /**
      * Is this input means interactive?
diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index c9b3be229ab0..ec0800947867 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -70,12 +70,12 @@
  */
 class Installer
 {
-    const ERROR_NONE = 0; // no error/success state
-    const ERROR_GENERIC_FAILURE = 1;
-    const ERROR_NO_LOCK_FILE_FOR_PARTIAL_UPDATE = 3;
-    const ERROR_LOCK_FILE_INVALID = 4;
+    public const ERROR_NONE = 0; // no error/success state
+    public const ERROR_GENERIC_FAILURE = 1;
+    public const ERROR_NO_LOCK_FILE_FOR_PARTIAL_UPDATE = 3;
+    public const ERROR_LOCK_FILE_INVALID = 4;
     // used/declared in SolverProblemsException, carried over here for completeness
-    const ERROR_DEPENDENCY_RESOLUTION_FAILED = 2;
+    public const ERROR_DEPENDENCY_RESOLUTION_FAILED = 2;
 
     /**
      * @var IOInterface
diff --git a/src/Composer/Installer/InstallerEvents.php b/src/Composer/Installer/InstallerEvents.php
index f75b8bdd4171..d536b459cf61 100644
--- a/src/Composer/Installer/InstallerEvents.php
+++ b/src/Composer/Installer/InstallerEvents.php
@@ -22,5 +22,5 @@ class InstallerEvents
      *
      * @var string
      */
-    const PRE_OPERATIONS_EXEC = 'pre-operations-exec';
+    public const PRE_OPERATIONS_EXEC = 'pre-operations-exec';
 }
diff --git a/src/Composer/Installer/PackageEvents.php b/src/Composer/Installer/PackageEvents.php
index 1b48e4f696f9..852da58ff126 100644
--- a/src/Composer/Installer/PackageEvents.php
+++ b/src/Composer/Installer/PackageEvents.php
@@ -26,7 +26,7 @@ class PackageEvents
      *
      * @var string
      */
-    const PRE_PACKAGE_INSTALL = 'pre-package-install';
+    public const PRE_PACKAGE_INSTALL = 'pre-package-install';
 
     /**
      * The POST_PACKAGE_INSTALL event occurs after a package is installed.
@@ -35,7 +35,7 @@ class PackageEvents
      *
      * @var string
      */
-    const POST_PACKAGE_INSTALL = 'post-package-install';
+    public const POST_PACKAGE_INSTALL = 'post-package-install';
 
     /**
      * The PRE_PACKAGE_UPDATE event occurs before a package is updated.
@@ -44,7 +44,7 @@ class PackageEvents
      *
      * @var string
      */
-    const PRE_PACKAGE_UPDATE = 'pre-package-update';
+    public const PRE_PACKAGE_UPDATE = 'pre-package-update';
 
     /**
      * The POST_PACKAGE_UPDATE event occurs after a package is updated.
@@ -53,7 +53,7 @@ class PackageEvents
      *
      * @var string
      */
-    const POST_PACKAGE_UPDATE = 'post-package-update';
+    public const POST_PACKAGE_UPDATE = 'post-package-update';
 
     /**
      * The PRE_PACKAGE_UNINSTALL event occurs before a package has been uninstalled.
@@ -62,7 +62,7 @@ class PackageEvents
      *
      * @var string
      */
-    const PRE_PACKAGE_UNINSTALL = 'pre-package-uninstall';
+    public const PRE_PACKAGE_UNINSTALL = 'pre-package-uninstall';
 
     /**
      * The POST_PACKAGE_UNINSTALL event occurs after a package has been uninstalled.
@@ -71,5 +71,5 @@ class PackageEvents
      *
      * @var string
      */
-    const POST_PACKAGE_UNINSTALL = 'post-package-uninstall';
+    public const POST_PACKAGE_UNINSTALL = 'post-package-uninstall';
 }
diff --git a/src/Composer/Installer/SuggestedPackagesReporter.php b/src/Composer/Installer/SuggestedPackagesReporter.php
index 3b1c92651b32..f2cfafebda7d 100644
--- a/src/Composer/Installer/SuggestedPackagesReporter.php
+++ b/src/Composer/Installer/SuggestedPackagesReporter.php
@@ -25,9 +25,9 @@
  */
 class SuggestedPackagesReporter
 {
-    const MODE_LIST = 1;
-    const MODE_BY_PACKAGE = 2;
-    const MODE_BY_SUGGESTION = 4;
+    public const MODE_LIST = 1;
+    public const MODE_BY_PACKAGE = 2;
+    public const MODE_BY_SUGGESTION = 4;
 
     /**
      * @var array<array{source: string, target: string, reason: string}>
diff --git a/src/Composer/Json/JsonFile.php b/src/Composer/Json/JsonFile.php
index a14ba7b60bc1..d464d9e77c35 100644
--- a/src/Composer/Json/JsonFile.php
+++ b/src/Composer/Json/JsonFile.php
@@ -28,8 +28,8 @@
  */
 class JsonFile
 {
-    const LAX_SCHEMA = 1;
-    const STRICT_SCHEMA = 2;
+    public const LAX_SCHEMA = 1;
+    public const STRICT_SCHEMA = 2;
 
     /** @deprecated Use \JSON_UNESCAPED_SLASHES */
     public const JSON_UNESCAPED_SLASHES = 64;
@@ -38,7 +38,7 @@ class JsonFile
     /** @deprecated Use \JSON_UNESCAPED_UNICODE */
     public const JSON_UNESCAPED_UNICODE = 256;
 
-    const COMPOSER_SCHEMA_PATH = '/../../../res/composer-schema.json';
+    public const COMPOSER_SCHEMA_PATH = __DIR__ . '/../../../res/composer-schema.json';
 
     /** @var string */
     private $path;
@@ -204,7 +204,7 @@ public function validateSchema(int $schema = self::STRICT_SCHEMA, ?string $schem
         $isComposerSchemaFile = false;
         if (null === $schemaFile) {
             $isComposerSchemaFile = true;
-            $schemaFile = __DIR__ . self::COMPOSER_SCHEMA_PATH;
+            $schemaFile = self::COMPOSER_SCHEMA_PATH;
         }
 
         // Prepend with file:// only when not using a special schema already (e.g. in the phar)
diff --git a/src/Composer/Package/BasePackage.php b/src/Composer/Package/BasePackage.php
index 1adff603b12e..d673caf05911 100644
--- a/src/Composer/Package/BasePackage.php
+++ b/src/Composer/Package/BasePackage.php
@@ -34,11 +34,11 @@ abstract class BasePackage implements PackageInterface
         'require-dev' => array('description' => 'requires (for development)', 'method' => Link::TYPE_DEV_REQUIRE),
     );
 
-    const STABILITY_STABLE = 0;
-    const STABILITY_RC = 5;
-    const STABILITY_BETA = 10;
-    const STABILITY_ALPHA = 15;
-    const STABILITY_DEV = 20;
+    public const STABILITY_STABLE = 0;
+    public const STABILITY_RC = 5;
+    public const STABILITY_BETA = 10;
+    public const STABILITY_ALPHA = 15;
+    public const STABILITY_DEV = 20;
 
     /** @var array<string, self::STABILITY_*> */
     public static $stabilities = array(
diff --git a/src/Composer/Package/Link.php b/src/Composer/Package/Link.php
index 481988c154f1..1430cd800715 100644
--- a/src/Composer/Package/Link.php
+++ b/src/Composer/Package/Link.php
@@ -32,10 +32,7 @@ class Link
      * @internal
      */
     public const TYPE_DOES_NOT_REQUIRE = 'does not require';
-    /**
-     * TODO should be marked private once 5.3 is dropped
-     * @private
-     */
+
     private const TYPE_UNKNOWN = 'relates to';
 
     /**
diff --git a/src/Composer/Package/Loader/LoaderInterface.php b/src/Composer/Package/Loader/LoaderInterface.php
index c81176ded91d..be2e4704237b 100644
--- a/src/Composer/Package/Loader/LoaderInterface.php
+++ b/src/Composer/Package/Loader/LoaderInterface.php
@@ -12,7 +12,6 @@
 
 namespace Composer\Package\Loader;
 
-use Composer\Package\CompletePackageInterface;
 use Composer\Package\CompletePackage;
 use Composer\Package\CompleteAliasPackage;
 use Composer\Package\RootAliasPackage;
diff --git a/src/Composer/Package/Loader/ValidatingArrayLoader.php b/src/Composer/Package/Loader/ValidatingArrayLoader.php
index 70a6affe3008..bf51849fd0ad 100644
--- a/src/Composer/Package/Loader/ValidatingArrayLoader.php
+++ b/src/Composer/Package/Loader/ValidatingArrayLoader.php
@@ -24,9 +24,9 @@
  */
 class ValidatingArrayLoader implements LoaderInterface
 {
-    const CHECK_ALL = 3;
-    const CHECK_UNBOUND_CONSTRAINTS = 1;
-    const CHECK_STRICT_CONSTRAINTS = 2;
+    public const CHECK_ALL = 3;
+    public const CHECK_UNBOUND_CONSTRAINTS = 1;
+    public const CHECK_STRICT_CONSTRAINTS = 2;
 
     /** @var LoaderInterface */
     private $loader;
diff --git a/src/Composer/Package/PackageInterface.php b/src/Composer/Package/PackageInterface.php
index 31a0e78fb8f9..e666e7139d0a 100644
--- a/src/Composer/Package/PackageInterface.php
+++ b/src/Composer/Package/PackageInterface.php
@@ -26,9 +26,9 @@
  */
 interface PackageInterface
 {
-    const DISPLAY_SOURCE_REF_IF_DEV = 0;
-    const DISPLAY_SOURCE_REF = 1;
-    const DISPLAY_DIST_REF = 2;
+    public const DISPLAY_SOURCE_REF_IF_DEV = 0;
+    public const DISPLAY_SOURCE_REF = 1;
+    public const DISPLAY_DIST_REF = 2;
 
     /**
      * Returns the package's name without version info, thus not a unique identifier
diff --git a/src/Composer/Package/RootPackage.php b/src/Composer/Package/RootPackage.php
index 07bd31d6fe87..a0b7ec950516 100644
--- a/src/Composer/Package/RootPackage.php
+++ b/src/Composer/Package/RootPackage.php
@@ -19,7 +19,7 @@
  */
 class RootPackage extends CompletePackage implements RootPackageInterface
 {
-    const DEFAULT_PRETTY_VERSION = '1.0.0+no-version-set';
+    public const DEFAULT_PRETTY_VERSION = '1.0.0+no-version-set';
 
     /** @var string */
     protected $minimumStability = 'stable';
diff --git a/src/Composer/Package/Version/VersionParser.php b/src/Composer/Package/Version/VersionParser.php
index faa5002c785c..6c0345999069 100644
--- a/src/Composer/Package/Version/VersionParser.php
+++ b/src/Composer/Package/Version/VersionParser.php
@@ -20,7 +20,7 @@
 
 class VersionParser extends SemverVersionParser
 {
-    const DEFAULT_BRANCH_ALIAS = '9999999-dev';
+    public const DEFAULT_BRANCH_ALIAS = '9999999-dev';
 
     /** @var array<string, ConstraintInterface> Constraint parsing cache */
     private static $constraints = array();
diff --git a/src/Composer/Plugin/PluginEvents.php b/src/Composer/Plugin/PluginEvents.php
index ff476ba40d17..c2e58d510f36 100644
--- a/src/Composer/Plugin/PluginEvents.php
+++ b/src/Composer/Plugin/PluginEvents.php
@@ -27,7 +27,7 @@ class PluginEvents
      *
      * @var string
      */
-    const INIT = 'init';
+    public const INIT = 'init';
 
     /**
      * The COMMAND event occurs as a command begins
@@ -37,7 +37,7 @@ class PluginEvents
      *
      * @var string
      */
-    const COMMAND = 'command';
+    public const COMMAND = 'command';
 
     /**
      * The PRE_FILE_DOWNLOAD event occurs before downloading a file
@@ -47,7 +47,7 @@ class PluginEvents
      *
      * @var string
      */
-    const PRE_FILE_DOWNLOAD = 'pre-file-download';
+    public const PRE_FILE_DOWNLOAD = 'pre-file-download';
 
     /**
      * The POST_FILE_DOWNLOAD event occurs after downloading a package dist file
@@ -57,7 +57,7 @@ class PluginEvents
      *
      * @var string
      */
-    const POST_FILE_DOWNLOAD = 'post-file-download';
+    public const POST_FILE_DOWNLOAD = 'post-file-download';
 
     /**
      * The PRE_COMMAND_RUN event occurs before a command is executed and lets you modify the input arguments/options
@@ -67,7 +67,7 @@ class PluginEvents
      *
      * @var string
      */
-    const PRE_COMMAND_RUN = 'pre-command-run';
+    public const PRE_COMMAND_RUN = 'pre-command-run';
 
     /**
      * The PRE_POOL_CREATE event occurs before the Pool of packages is created, and lets
@@ -78,5 +78,5 @@ class PluginEvents
      *
      * @var string
      */
-    const PRE_POOL_CREATE = 'pre-pool-create';
+    public const PRE_POOL_CREATE = 'pre-pool-create';
 }
diff --git a/src/Composer/Plugin/PluginInterface.php b/src/Composer/Plugin/PluginInterface.php
index c2a35e50adf8..ae5139a849d2 100644
--- a/src/Composer/Plugin/PluginInterface.php
+++ b/src/Composer/Plugin/PluginInterface.php
@@ -32,7 +32,7 @@ interface PluginInterface
      *
      * @var string
      */
-    const PLUGIN_API_VERSION = '2.3.0';
+    public const PLUGIN_API_VERSION = '2.3.0';
 
     /**
      * Apply plugin modifications to Composer
diff --git a/src/Composer/Repository/PlatformRepository.php b/src/Composer/Repository/PlatformRepository.php
index 461191e4a2ae..3c9c625fed86 100644
--- a/src/Composer/Repository/PlatformRepository.php
+++ b/src/Composer/Repository/PlatformRepository.php
@@ -32,7 +32,11 @@
  */
 class PlatformRepository extends ArrayRepository
 {
-    const PLATFORM_PACKAGE_REGEX = '{^(?:php(?:-64bit|-ipv6|-zts|-debug)?|hhvm|(?:ext|lib)-[a-z0-9](?:[_.-]?[a-z0-9]+)*|composer(?:-(?:plugin|runtime)-api)?)$}iD';
+    /**
+     * @deprecated use PlatformRepository::isPlatformPackage(string $name) instead
+     * @private
+     */
+    public const PLATFORM_PACKAGE_REGEX = '{^(?:php(?:-64bit|-ipv6|-zts|-debug)?|hhvm|(?:ext|lib)-[a-z0-9](?:[_.-]?[a-z0-9]+)*|composer(?:-(?:plugin|runtime)-api)?)$}iD';
 
     /**
      * @var ?string
diff --git a/src/Composer/Repository/RepositoryInterface.php b/src/Composer/Repository/RepositoryInterface.php
index 0de31edd3d64..129a0949879a 100644
--- a/src/Composer/Repository/RepositoryInterface.php
+++ b/src/Composer/Repository/RepositoryInterface.php
@@ -25,9 +25,9 @@
  */
 interface RepositoryInterface extends \Countable
 {
-    const SEARCH_FULLTEXT = 0;
-    const SEARCH_NAME = 1;
-    const SEARCH_VENDOR = 2;
+    public const SEARCH_FULLTEXT = 0;
+    public const SEARCH_NAME = 1;
+    public const SEARCH_VENDOR = 2;
 
     /**
      * Checks if specified package registered (installed).
diff --git a/src/Composer/Repository/RepositorySet.php b/src/Composer/Repository/RepositorySet.php
index d13a423e03ca..c0e39fd5f2dd 100644
--- a/src/Composer/Repository/RepositorySet.php
+++ b/src/Composer/Repository/RepositorySet.php
@@ -34,11 +34,11 @@ class RepositorySet
     /**
      * Packages are returned even though their stability does not match the required stability
      */
-    const ALLOW_UNACCEPTABLE_STABILITIES = 1;
+    public const ALLOW_UNACCEPTABLE_STABILITIES = 1;
     /**
      * Packages will be looked up in all repositories, even after they have been found in a higher prio one
      */
-    const ALLOW_SHADOWED_REPOSITORIES = 2;
+    public const ALLOW_SHADOWED_REPOSITORIES = 2;
 
     /**
      * @var array[]
diff --git a/src/Composer/Repository/Vcs/GitLabDriver.php b/src/Composer/Repository/Vcs/GitLabDriver.php
index 047cefa240a5..13ecd755d636 100644
--- a/src/Composer/Repository/Vcs/GitLabDriver.php
+++ b/src/Composer/Repository/Vcs/GitLabDriver.php
@@ -82,7 +82,7 @@ class GitLabDriver extends VcsDriver
      */
     private $hasNonstandardOrigin = false;
 
-    const URL_REGEX = '#^(?:(?P<scheme>https?)://(?P<domain>.+?)(?::(?P<port>[0-9]+))?/|git@(?P<domain2>[^:]+):)(?P<parts>.+)/(?P<repo>[^/]+?)(?:\.git|/)?$#';
+    private const URL_REGEX = '#^(?:(?P<scheme>https?)://(?P<domain>.+?)(?::(?P<port>[0-9]+))?/|git@(?P<domain2>[^:]+):)(?P<parts>.+)/(?P<repo>[^/]+?)(?:\.git|/)?$#';
 
     /**
      * Extracts information from the repository url.
diff --git a/src/Composer/Script/ScriptEvents.php b/src/Composer/Script/ScriptEvents.php
index d2dc6662554f..33cc4f57dd8b 100644
--- a/src/Composer/Script/ScriptEvents.php
+++ b/src/Composer/Script/ScriptEvents.php
@@ -27,7 +27,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const PRE_INSTALL_CMD = 'pre-install-cmd';
+    public const PRE_INSTALL_CMD = 'pre-install-cmd';
 
     /**
      * The POST_INSTALL_CMD event occurs after the install command is executed.
@@ -36,7 +36,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_INSTALL_CMD = 'post-install-cmd';
+    public const POST_INSTALL_CMD = 'post-install-cmd';
 
     /**
      * The PRE_UPDATE_CMD event occurs before the update command is executed.
@@ -45,7 +45,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const PRE_UPDATE_CMD = 'pre-update-cmd';
+    public const PRE_UPDATE_CMD = 'pre-update-cmd';
 
     /**
      * The POST_UPDATE_CMD event occurs after the update command is executed.
@@ -54,7 +54,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_UPDATE_CMD = 'post-update-cmd';
+    public const POST_UPDATE_CMD = 'post-update-cmd';
 
     /**
      * The PRE_STATUS_CMD event occurs before the status command is executed.
@@ -63,7 +63,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const PRE_STATUS_CMD = 'pre-status-cmd';
+    public const PRE_STATUS_CMD = 'pre-status-cmd';
 
     /**
      * The POST_STATUS_CMD event occurs after the status command is executed.
@@ -72,7 +72,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_STATUS_CMD = 'post-status-cmd';
+    public const POST_STATUS_CMD = 'post-status-cmd';
 
     /**
      * The PRE_AUTOLOAD_DUMP event occurs before the autoload file is generated.
@@ -81,7 +81,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const PRE_AUTOLOAD_DUMP = 'pre-autoload-dump';
+    public const PRE_AUTOLOAD_DUMP = 'pre-autoload-dump';
 
     /**
      * The POST_AUTOLOAD_DUMP event occurs after the autoload file has been generated.
@@ -90,7 +90,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_AUTOLOAD_DUMP = 'post-autoload-dump';
+    public const POST_AUTOLOAD_DUMP = 'post-autoload-dump';
 
     /**
      * The POST_ROOT_PACKAGE_INSTALL event occurs after the root package has been installed.
@@ -99,7 +99,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_ROOT_PACKAGE_INSTALL = 'post-root-package-install';
+    public const POST_ROOT_PACKAGE_INSTALL = 'post-root-package-install';
 
     /**
      * The POST_CREATE_PROJECT event occurs after the create-project command has been executed.
@@ -109,7 +109,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_CREATE_PROJECT_CMD = 'post-create-project-cmd';
+    public const POST_CREATE_PROJECT_CMD = 'post-create-project-cmd';
 
     /**
      * The PRE_ARCHIVE_CMD event occurs before the update command is executed.
@@ -118,7 +118,7 @@ class ScriptEvents
      *
      * @var string
      */
-    const PRE_ARCHIVE_CMD = 'pre-archive-cmd';
+    public const PRE_ARCHIVE_CMD = 'pre-archive-cmd';
 
     /**
      * The POST_ARCHIVE_CMD event occurs after the status command is executed.
@@ -127,5 +127,5 @@ class ScriptEvents
      *
      * @var string
      */
-    const POST_ARCHIVE_CMD = 'post-archive-cmd';
+    public const POST_ARCHIVE_CMD = 'post-archive-cmd';
 }
diff --git a/src/Composer/Util/Bitbucket.php b/src/Composer/Util/Bitbucket.php
index 4ce3b7599ddb..b80eb37bec79 100644
--- a/src/Composer/Util/Bitbucket.php
+++ b/src/Composer/Util/Bitbucket.php
@@ -35,7 +35,7 @@ class Bitbucket
     /** @var int|null */
     private $time;
 
-    const OAUTH2_ACCESS_TOKEN_URL = 'https://bitbucket.org/site/oauth2/access_token';
+    public const OAUTH2_ACCESS_TOKEN_URL = 'https://bitbucket.org/site/oauth2/access_token';
 
     /**
      * Constructor.
diff --git a/src/Composer/Util/ConfigValidator.php b/src/Composer/Util/ConfigValidator.php
index 4d2f50d2dd09..ea84c10b299b 100644
--- a/src/Composer/Util/ConfigValidator.php
+++ b/src/Composer/Util/ConfigValidator.php
@@ -29,7 +29,7 @@
  */
 class ConfigValidator
 {
-    const CHECK_VERSION = 1;
+    public const CHECK_VERSION = 1;
 
     /** @var IOInterface */
     private $io;
diff --git a/src/Composer/Util/HttpDownloader.php b/src/Composer/Util/HttpDownloader.php
index bc229960f3e4..b0377bf79d7a 100644
--- a/src/Composer/Util/HttpDownloader.php
+++ b/src/Composer/Util/HttpDownloader.php
@@ -32,11 +32,11 @@
  */
 class HttpDownloader
 {
-    const STATUS_QUEUED = 1;
-    const STATUS_STARTED = 2;
-    const STATUS_COMPLETED = 3;
-    const STATUS_FAILED = 4;
-    const STATUS_ABORTED = 5;
+    private const STATUS_QUEUED = 1;
+    private const STATUS_STARTED = 2;
+    private const STATUS_COMPLETED = 3;
+    private const STATUS_FAILED = 4;
+    private const STATUS_ABORTED = 5;
 
     /** @var IOInterface */
     private $io;
diff --git a/src/Composer/Util/ProcessExecutor.php b/src/Composer/Util/ProcessExecutor.php
index f55515192688..f936fcad116f 100644
--- a/src/Composer/Util/ProcessExecutor.php
+++ b/src/Composer/Util/ProcessExecutor.php
@@ -25,11 +25,11 @@
  */
 class ProcessExecutor
 {
-    const STATUS_QUEUED = 1;
-    const STATUS_STARTED = 2;
-    const STATUS_COMPLETED = 3;
-    const STATUS_FAILED = 4;
-    const STATUS_ABORTED = 5;
+    private const STATUS_QUEUED = 1;
+    private const STATUS_STARTED = 2;
+    private const STATUS_COMPLETED = 3;
+    private const STATUS_FAILED = 4;
+    private const STATUS_ABORTED = 5;
 
     /** @var int */
     protected static $timeout = 300;
diff --git a/src/Composer/Util/Svn.php b/src/Composer/Util/Svn.php
index 86416e56d508..00a090d5c2c9 100644
--- a/src/Composer/Util/Svn.php
+++ b/src/Composer/Util/Svn.php
@@ -22,7 +22,7 @@
  */
 class Svn
 {
-    const MAX_QTY_AUTH_TRIES = 5;
+    private const MAX_QTY_AUTH_TRIES = 5;
 
     /**
      * @var ?array{username: string, password: string}
diff --git a/src/bootstrap.php b/src/bootstrap.php
index f61063a4a0f8..3b2cb4d852f7 100644
--- a/src/bootstrap.php
+++ b/src/bootstrap.php
@@ -10,7 +10,9 @@
  * file that was distributed with this source code.
  */
 
-function includeIfExists(string $file): ?\Composer\Autoload\ClassLoader
+use Composer\Autoload\ClassLoader;
+
+function includeIfExists(string $file): ?ClassLoader
 {
     return file_exists($file) ? include $file : null;
 }
diff --git a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
index 87ff8c4aaa71..508f1c03a0f3 100644
--- a/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
+++ b/tests/Composer/Test/Autoload/AutoloadGeneratorTest.php
@@ -83,12 +83,9 @@ class AutoloadGeneratorTest extends TestCase
      * Map of setting name => return value configuration for the stub Config
      * object.
      *
-     * Note: must be public for compatibility with PHP 5.3 runtimes where
-     * closures cannot access private members of the classes they are created
-     * in.
      * @var array<string, callable|boolean>
      */
-    public $configValueMap;
+    private $configValueMap;
 
     protected function setUp(): void
     {
diff --git a/tests/Composer/Test/Command/InitCommandTest.php b/tests/Composer/Test/Command/InitCommandTest.php
index 25af49ed503c..f33f730b93a1 100644
--- a/tests/Composer/Test/Command/InitCommandTest.php
+++ b/tests/Composer/Test/Command/InitCommandTest.php
@@ -66,7 +66,8 @@ public function testParseNumericAuthorString(): void
     public function testParseValidAlias1AuthorString(): void
     {
         $command = new InitCommand;
-        $author = $this->callParseAuthorString($command,
+        $author = $this->callParseAuthorString(
+            $command,
             'Johnathon "Johnny" Smith <john@example.com>'
         );
         $this->assertEquals('Johnathon "Johnny" Smith', $author['name']);
@@ -80,7 +81,8 @@ public function testParseValidAlias1AuthorString(): void
     public function testParseValidAlias2AuthorString(): void
     {
         $command = new InitCommand;
-        $author = $this->callParseAuthorString($command,
+        $author = $this->callParseAuthorString(
+            $command,
             'Johnathon (Johnny) Smith <john@example.com>'
         );
         $this->assertEquals('Johnathon (Johnny) Smith', $author['name']);
diff --git a/tests/Composer/Test/Repository/PlatformRepositoryTest.php b/tests/Composer/Test/Repository/PlatformRepositoryTest.php
index 22445b4df459..18c2a68d8795 100644
--- a/tests/Composer/Test/Repository/PlatformRepositoryTest.php
+++ b/tests/Composer/Test/Repository/PlatformRepositoryTest.php
@@ -1271,7 +1271,7 @@ public function testValidPlatformPackages(string $packageName, bool $expectation
 
 class ResourceBundleStub
 {
-    const STUB_VERSION = '32.0.1';
+    public const STUB_VERSION = '32.0.1';
 
     /**
      * @param string $locale
diff --git a/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php b/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
index f1787e71c271..fda883a43825 100644
--- a/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
+++ b/tests/Composer/Test/Repository/Vcs/PerforceDriverTest.php
@@ -57,9 +57,9 @@ class PerforceDriverTest extends TestCase
      */
     protected $perforce;
 
-    const TEST_URL = 'TEST_PERFORCE_URL';
-    const TEST_DEPOT = 'TEST_DEPOT_CONFIG';
-    const TEST_BRANCH = 'TEST_BRANCH_CONFIG';
+    private const TEST_URL = 'TEST_PERFORCE_URL';
+    private const TEST_DEPOT = 'TEST_DEPOT_CONFIG';
+    private const TEST_BRANCH = 'TEST_BRANCH_CONFIG';
 
     protected function setUp(): void
     {
diff --git a/tests/Composer/Test/Util/PerforceTest.php b/tests/Composer/Test/Util/PerforceTest.php
index b42f484b36f5..7b524337f187 100644
--- a/tests/Composer/Test/Util/PerforceTest.php
+++ b/tests/Composer/Test/Util/PerforceTest.php
@@ -32,12 +32,12 @@ class PerforceTest extends TestCase
     /** @var \PHPUnit\Framework\MockObject\MockObject&\Composer\IO\IOInterface */
     protected $io;
 
-    const TEST_DEPOT = 'depot';
-    const TEST_BRANCH = 'branch';
-    const TEST_P4USER = 'user';
-    const TEST_CLIENT_NAME = 'TEST';
-    const TEST_PORT = 'port';
-    const TEST_PATH = 'path';
+    private const TEST_DEPOT = 'depot';
+    private const TEST_BRANCH = 'branch';
+    private const TEST_P4USER = 'user';
+    private const TEST_CLIENT_NAME = 'TEST';
+    private const TEST_PORT = 'port';
+    private const TEST_PATH = 'path';
 
     protected function setUp(): void
     {