diff --git a/resources/schema.json b/resources/schema.json
index 5a37a82f4..14b93a998 100644
--- a/resources/schema.json
+++ b/resources/schema.json
@@ -144,6 +144,81 @@
                             }
                         ]
                     }
+                },
+                "MBString": {
+                    "type": {
+                        "anyOf": [
+                            {
+                                "type": "boolean"
+                            },
+                            {
+                                "type": "object",
+                                "additionalProperties": false,
+                                "properties": {
+                                    "settings": {
+                                        "type": "object",
+                                        "additionalProperties": false,
+                                        "properties": {
+                                            "mb_chr": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_ord": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_parse_str": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_send_mail": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strcut": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_stripos": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_stristr": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strlen": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strpos": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strrchr": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strripos": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strrpos": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strstr": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strtolower": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_strtoupper": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_substr_count": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_substr": {
+                                                "type": "boolean"
+                                            },
+                                            "mb_convert_case": {
+                                                "type": "boolean"
+                                            }
+                                        }
+                                    }
+                                }
+                            }
+                        ]
+                    }
                 }
             }
         },
diff --git a/src/Mutator/Extensions/MBString.php b/src/Mutator/Extensions/MBString.php
new file mode 100644
index 000000000..c10fc4417
--- /dev/null
+++ b/src/Mutator/Extensions/MBString.php
@@ -0,0 +1,196 @@
+<?php
+/**
+ * This code is licensed under the BSD 3-Clause License.
+ *
+ * Copyright (c) 2017-2019, Maks Rafalko
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ *
+ * * Neither the name of the copyright holder nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+declare(strict_types=1);
+
+namespace Infection\Mutator\Extensions;
+
+use Generator;
+use Infection\Mutator\Util\Mutator;
+use Infection\Mutator\Util\MutatorConfig;
+use PhpParser\Node;
+
+/**
+ * @internal
+ */
+final class MBString extends Mutator
+{
+    private $converters;
+
+    public function __construct(MutatorConfig $config)
+    {
+        parent::__construct($config);
+
+        $settings = $this->getSettings();
+
+        $this->setupConverters($settings);
+    }
+
+    /**
+     * @param Node\Expr\FuncCall $node
+     *
+     * @return Node|Node[]|Generator
+     */
+    public function mutate(Node $node)
+    {
+        yield from $this->converters[$node->name->toLowerString()]($node);
+    }
+
+    protected function mutatesNode(Node $node): bool
+    {
+        if (!$node instanceof Node\Expr\FuncCall || !$node->name instanceof Node\Name) {
+            return false;
+        }
+
+        return isset($this->converters[$node->name->toLowerString()]);
+    }
+
+    private function setupConverters(array $functionsMap): void
+    {
+        $converters = [
+            'mb_chr' => $this->mapFunctionAndRemoveExtraArgs('chr', 1),
+            'mb_ord' => $this->mapFunctionAndRemoveExtraArgs('ord', 1),
+            'mb_parse_str' => $this->mapFunction('parse_str'),
+            'mb_send_mail' => $this->mapFunction('mail'),
+            'mb_strcut' => $this->mapFunctionAndRemoveExtraArgs('substr', 3),
+            'mb_stripos' => $this->mapFunctionAndRemoveExtraArgs('stripos', 3),
+            'mb_stristr' => $this->mapFunctionAndRemoveExtraArgs('stristr', 3),
+            'mb_strlen' => $this->mapFunctionAndRemoveExtraArgs('strlen', 1),
+            'mb_strpos' => $this->mapFunctionAndRemoveExtraArgs('strpos', 3),
+            'mb_strrchr' => $this->mapFunctionAndRemoveExtraArgs('strrchr', 2),
+            'mb_strripos' => $this->mapFunctionAndRemoveExtraArgs('strripos', 3),
+            'mb_strrpos' => $this->mapFunctionAndRemoveExtraArgs('strrpos', 3),
+            'mb_strstr' => $this->mapFunctionAndRemoveExtraArgs('strstr', 3),
+            'mb_strtolower' => $this->mapFunctionAndRemoveExtraArgs('strtolower', 1),
+            'mb_strtoupper' => $this->mapFunctionAndRemoveExtraArgs('strtoupper', 1),
+            'mb_substr_count' => $this->mapFunctionAndRemoveExtraArgs('substr_count', 2),
+            'mb_substr' => $this->mapFunctionAndRemoveExtraArgs('substr', 3),
+            'mb_convert_case' => $this->mapConvertCase(),
+        ];
+
+        $functionsToRemove = \array_filter($functionsMap, static function ($isOn) {
+            return !$isOn;
+        });
+
+        $this->converters = \array_diff_key($converters, $functionsToRemove);
+    }
+
+    private function mapFunction(string $newFunctionName): callable
+    {
+        return function (Node\Expr\FuncCall $node) use ($newFunctionName): Generator {
+            yield $this->mapFunctionCall($node, $newFunctionName, $node->args);
+        };
+    }
+
+    private function mapFunctionAndRemoveExtraArgs(string $newFunctionName, int $argsAtMost): callable
+    {
+        return function (Node\Expr\FuncCall $node) use ($newFunctionName, $argsAtMost): Generator {
+            yield $this->mapFunctionCall($node, $newFunctionName, \array_slice($node->args, 0, $argsAtMost));
+        };
+    }
+
+    private function mapConvertCase(): callable
+    {
+        return function (Node\Expr\FuncCall $node): Generator {
+            $modeValue = $this->getConvertCaseModeValue($node);
+
+            if ($modeValue === null) {
+                return;
+            }
+
+            $functionName = $this->getConvertCaseFunctionName($modeValue);
+
+            if ($functionName === null) {
+                return;
+            }
+
+            yield $this->mapFunctionCall($node, $functionName, [$node->args[0]]);
+        };
+    }
+
+    private function getConvertCaseModeValue(Node\Expr\FuncCall $node): ?int
+    {
+        if (\count($node->args) < 2) {
+            return null;
+        }
+
+        $mode = $node->args[1]->value;
+
+        if ($mode instanceof Node\Scalar\LNumber) {
+            return $mode->value;
+        }
+
+        if ($mode instanceof Node\Expr\ConstFetch) {
+            return \constant($mode->name->toString());
+        }
+
+        return null;
+    }
+
+    private function getConvertCaseFunctionName(int $mode): ?string
+    {
+        if ($this->isInMbCaseMode($mode, 'MB_CASE_UPPER', 'MB_CASE_UPPER_SIMPLE')) {
+            return 'strtoupper';
+        }
+
+        if ($this->isInMbCaseMode($mode, 'MB_CASE_LOWER', 'MB_CASE_LOWER_SIMPLE', 'MB_CASE_FOLD', 'MB_CASE_FOLD_SIMPLE')) {
+            return 'strtolower';
+        }
+
+        if ($this->isInMbCaseMode($mode, 'MB_CASE_TITLE', 'MB_CASE_TITLE_SIMPLE')) {
+            return 'ucwords';
+        }
+
+        return null;
+    }
+
+    private function isInMbCaseMode(int $mode, string ...$cases): bool
+    {
+        foreach ($cases as $constant) {
+            if (\defined($constant) && \constant($constant) === $mode) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    private function mapFunctionCall(Node\Expr\FuncCall $node, string $newFuncName, array $args): Node\Expr\FuncCall
+    {
+        return new Node\Expr\FuncCall(
+            new Node\Name($newFuncName, $node->name->getAttributes()),
+            $args,
+            $node->getAttributes()
+        );
+    }
+}
diff --git a/src/Mutator/Util/MutatorProfile.php b/src/Mutator/Util/MutatorProfile.php
index ba113192e..a8f46427c 100644
--- a/src/Mutator/Util/MutatorProfile.php
+++ b/src/Mutator/Util/MutatorProfile.php
@@ -60,6 +60,7 @@ final class MutatorProfile
         '@zero_iteration' => self::ZERO_ITERATION,
         '@cast' => self::CAST,
         '@unwrap' => self::UNWRAP,
+        '@extensions' => self::EXTENSIONS,
 
         //Special Profiles
         '@default' => self::DEFAULT,
@@ -232,6 +233,10 @@ final class MutatorProfile
         Mutator\Unwrap\UnwrapUcWords::class,
     ];
 
+    public const EXTENSIONS = [
+        Mutator\Extensions\MBString::class,
+    ];
+
     public const DEFAULT = [
         '@arithmetic',
         '@boolean',
@@ -246,6 +251,7 @@ final class MutatorProfile
         '@return_value',
         '@sort',
         '@zero_iteration',
+        '@extensions',
     ];
 
     public const FULL_MUTATOR_LIST = [
@@ -395,5 +401,8 @@ final class MutatorProfile
         'UnwrapTrim' => Mutator\Unwrap\UnwrapTrim::class,
         'UnwrapUcFirst' => Mutator\Unwrap\UnwrapUcFirst::class,
         'UnwrapUcWords' => Mutator\Unwrap\UnwrapUcWords::class,
+
+        // Extensions
+        'MBString' => Mutator\Extensions\MBString::class,
     ];
 }
diff --git a/tests/Mutator/Extensions/MBStringTest.php b/tests/Mutator/Extensions/MBStringTest.php
new file mode 100644
index 000000000..6a05a4f62
--- /dev/null
+++ b/tests/Mutator/Extensions/MBStringTest.php
@@ -0,0 +1,598 @@
+<?php
+/**
+ * This code is licensed under the BSD 3-Clause License.
+ *
+ * Copyright (c) 2017-2019, Maks Rafalko
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ *
+ * * Neither the name of the copyright holder nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+declare(strict_types=1);
+
+namespace Infection\Tests\Mutator\Extensions;
+
+use Generator;
+use Infection\Tests\Mutator\AbstractMutatorTestCase;
+
+/**
+ * @internal
+ */
+final class MBStringTest extends AbstractMutatorTestCase
+{
+    public static function setUpBeforeClass(): void
+    {
+        self::defineMissingMbCaseConstants();
+    }
+
+    /**
+     * @dataProvider provideMutationCases
+     */
+    public function test_mutator(string $input, string $expected = null, array $settings = []): void
+    {
+        $this->doTest($input, $expected, $settings);
+    }
+
+    public function provideMutationCases(): Generator
+    {
+        yield 'It converts mb_strlen with leading slash' => [
+            "<?php \mb_strlen('test');",
+            "<?php\n\nstrlen('test');",
+        ];
+
+        yield 'It converts mb_strlen with leading slash in namespace' => [
+            "<?php namespace Test; \mb_strlen('test');",
+            "<?php\n\nnamespace Test;\n\nstrlen('test');",
+        ];
+
+        yield 'It does not convert standard functions like strpos' => [
+            "<?php strpos('test');",
+        ];
+
+        yield 'It converts mb_strlen with encoding to strlen' => [
+            "<?php mb_strlen('test', 'utf-8');",
+            "<?php\n\nstrlen('test');",
+            ['settings' => ['mb_strlen' => true]],
+        ];
+
+        yield 'It does not convert mb_strlen when disabled' => [
+            "<?php mb_strlen('test');",
+            null,
+            ['settings' => ['mb_strlen' => false]],
+        ];
+
+        yield from $this->provideMutationCasesForChr();
+
+        yield from $this->provideMutationCasesForOrd();
+
+        yield from $this->provideMutationCasesForParseStr();
+
+        yield from $this->provideMutationCasesForSendMail();
+
+        yield from $this->provideMutationCasesForStrCut();
+
+        yield from $this->provideMutationCasesForStrPos();
+
+        yield from $this->provideMutationCasesForStrIPos();
+
+        yield from $this->provideMutationCasesForStrIStr();
+
+        yield from $this->provideMutationCasesForStrRiPos();
+
+        yield from $this->provideMutationCasesForStrRPos();
+
+        yield from $this->provideMutationCasesForStrStr();
+
+        yield from $this->provideMutationCasesForStrToLower();
+
+        yield from $this->provideMutationCasesForStrToUpper();
+
+        yield from $this->provideMutationCasesForSubStrCount();
+
+        yield from $this->provideMutationCasesForSubStr();
+
+        yield from $this->provideMutationCasesForStrRChr();
+
+        yield from $this->provideMutationCasesForConvertCase();
+    }
+
+    private function provideMutationCasesForChr(): Generator
+    {
+        yield 'It converts mb_chr to chr' => [
+            '<?php mb_chr(74);',
+            "<?php\n\nchr(74);",
+        ];
+
+        yield 'It converts correctly when mb_chr is wrongly capitalized' => [
+            '<?php mB_cHr(74);',
+            "<?php\n\nchr(74);",
+        ];
+
+        yield 'It converts mb_chr with encoding to chr' => [
+            "<?php mb_chr(74, 'utf-8');",
+            "<?php\n\nchr(74);",
+        ];
+
+        yield 'It does not mutate mb_chr called via variable' => [
+            '<?php $a = "mb_chr"; $a(74);',
+        ];
+    }
+
+    private function provideMutationCasesForOrd(): Generator
+    {
+        yield 'It converts mb_ord to ord' => [
+            "<?php mb_ord('T');",
+            "<?php\n\nord('T');",
+        ];
+
+        yield 'It converts correctly when mb_ord is wrongly capitalized' => [
+            "<?php MB_ord('T');",
+            "<?php\n\nord('T');",
+        ];
+
+        yield 'It converts mb_ord with encoding to ord' => [
+            "<?php mb_ord('T', 'utf-8');",
+            "<?php\n\nord('T');",
+        ];
+
+        yield 'It does not mutate mb_ord called via variable' => [
+            '<?php $a = "mb_ord"; $a("T");',
+        ];
+    }
+
+    private function provideMutationCasesForParseStr(): Generator
+    {
+        yield 'It converts mb_parse_str to parse_str' => [
+            "<?php mb_parse_str('T');",
+            "<?php\n\nparse_str('T');",
+        ];
+
+        yield 'It converts correctly when mb_parse_str is wrongly capitalize' => [
+            "<?php mb_pARse_Str('T');",
+            "<?php\n\nparse_str('T');",
+        ];
+
+        yield 'It converts mb_parse_str with params argument to parse_str' => [
+            "<?php mb_parse_str('T', \$params);",
+            "<?php\n\nparse_str('T', \$params);",
+        ];
+
+        yield 'It does not mutate mb_parse_str called via variable' => [
+            '<?php $a = "mb_parse_str"; $a("T");',
+        ];
+    }
+
+    private function provideMutationCasesForSendMail(): Generator
+    {
+        yield 'It converts mb_send_mail to mail' => [
+            "<?php mb_send_mail('to', 'subject', 'msg');",
+            "<?php\n\nmail('to', 'subject', 'msg');",
+        ];
+
+        yield 'It converts correctly when mb_send_mail is wrongly capitalize' => [
+            "<?php mb_SEND_mail('to', 'subject', 'msg');",
+            "<?php\n\nmail('to', 'subject', 'msg');",
+        ];
+
+        yield 'It converts mb_send_mail with additional parameters to mail' => [
+            "<?php mb_send_mail('to', 'subject', 'msg', [], []);",
+            "<?php\n\nmail('to', 'subject', 'msg', [], []);",
+        ];
+
+        yield 'It does not mutate mb_send_mail called via variable' => [
+            '<?php $a = "mb_send_mail"; $a("to", "subject", "msg");',
+        ];
+    }
+
+    private function provideMutationCasesForStrCut(): Generator
+    {
+        yield 'It converts mb_strcut to substr' => [
+            "<?php mb_strcut('subject', 1);",
+            "<?php\n\nsubstr('subject', 1);",
+        ];
+
+        yield 'It converts correctly when mb_strcut is wrongly capitalize' => [
+            "<?php MB_strcut('subject', 1);",
+            "<?php\n\nsubstr('subject', 1);",
+        ];
+
+        yield 'It converts mb_strcut with limit to substr' => [
+            "<?php mb_strcut('subject', 1, 20);",
+            "<?php\n\nsubstr('subject', 1, 20);",
+        ];
+
+        yield 'It converts mb_strcut with encoding to substr' => [
+            "<?php mb_strcut('subject', 1, 20, 'utf-8');",
+            "<?php\n\nsubstr('subject', 1, 20);",
+        ];
+
+        yield 'It does not mutate mb_strcut called via variable' => [
+            '<?php $a = "mb_strcut"; $a("subject", 1);',
+        ];
+    }
+
+    private function provideMutationCasesForStrPos(): Generator
+    {
+        yield 'It converts mb_strpos to strpos' => [
+            "<?php mb_strpos('subject', 'b');",
+            "<?php\n\nstrpos('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_strpos is wrongly capitalize' => [
+            "<?php mb_StRpOs('subject', 'b');",
+            "<?php\n\nstrpos('subject', 'b');",
+        ];
+
+        yield 'It converts mb_strpos with offset to strpos' => [
+            "<?php mb_strpos('subject', 'b', 3);",
+            "<?php\n\nstrpos('subject', 'b', 3);",
+        ];
+
+        yield 'It converts mb_strpos with encoding to strpos' => [
+            "<?php mb_strpos('subject', 'b', 3, 'utf-8');",
+            "<?php\n\nstrpos('subject', 'b', 3);",
+        ];
+
+        yield 'It does not mutate mb_strpos called via variable' => [
+            '<?php $a = "mb_strpos"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForStrIPos(): Generator
+    {
+        yield 'It converts mb_stripos to stripos' => [
+            "<?php mb_stripos('subject', 'b');",
+            "<?php\n\nstripos('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_stripos is wrongly capitalize' => [
+            "<?php mB_sTRIpos('subject', 'b');",
+            "<?php\n\nstripos('subject', 'b');",
+        ];
+
+        yield 'It converts mb_stripos with offset to stripos' => [
+            "<?php mb_stripos('subject', 'b', 3);",
+            "<?php\n\nstripos('subject', 'b', 3);",
+        ];
+
+        yield 'It converts mb_stripos with encoding to stripos' => [
+            "<?php mb_stripos('subject', 'b', 3, 'utf-8');",
+            "<?php\n\nstripos('subject', 'b', 3);",
+        ];
+
+        yield 'It does not mutate mb_stripos called via variable' => [
+            '<?php $a = "mb_stripos"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForStrIStr(): Generator
+    {
+        yield 'It converts mb_stristr to stristr' => [
+            "<?php mb_stristr('subject', 'b');",
+            "<?php\n\nstristr('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_stristr is wrongly capitalize' => [
+            "<?php mb_strISTR('subject', 'b');",
+            "<?php\n\nstristr('subject', 'b');",
+        ];
+
+        yield 'It converts mb_stristr with part argument to stristr' => [
+            "<?php mb_stristr('subject', 'b', false);",
+            "<?php\n\nstristr('subject', 'b', false);",
+        ];
+
+        yield 'It converts mb_stristr with encoding to stristr' => [
+            "<?php mb_stristr('subject', 'b', false, 'utf-8');",
+            "<?php\n\nstristr('subject', 'b', false);",
+        ];
+
+        yield 'It does not mutate mb_stristr called via variable' => [
+            '<?php $a = "mb_stristr"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForStrRiPos(): Generator
+    {
+        yield 'It converts mb_strripos to strripos' => [
+            "<?php mb_strripos('subject', 'b');",
+            "<?php\n\nstrripos('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_strripos is wrongly capitalize' => [
+            "<?php MB_sTrRipos('subject', 'b');",
+            "<?php\n\nstrripos('subject', 'b');",
+        ];
+
+        yield 'It converts mb_strripos with offset argument to strripos' => [
+            "<?php mb_strripos('subject', 'b', 2);",
+            "<?php\n\nstrripos('subject', 'b', 2);",
+        ];
+
+        yield 'It converts mb_strripos with encoding to strripos' => [
+            "<?php mb_strripos('subject', 'b', 2, 'utf-8');",
+            "<?php\n\nstrripos('subject', 'b', 2);",
+        ];
+
+        yield 'It does not mutate mb_strripos called via variable' => [
+            '<?php $a = "mb_strripos"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForStrRPos(): Generator
+    {
+        yield 'It converts mb_strrpos to strrpos' => [
+            "<?php mb_strrpos('subject', 'b');",
+            "<?php\n\nstrrpos('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_strrpos is wrongly capitalize' => [
+            "<?php mb_StRrPos('subject', 'b');",
+            "<?php\n\nstrrpos('subject', 'b');",
+        ];
+
+        yield 'It converts mb_strrpos with offset argument to strrpos' => [
+            "<?php mb_strrpos('subject', 'b', 2);",
+            "<?php\n\nstrrpos('subject', 'b', 2);",
+        ];
+
+        yield 'It converts mb_strrpos with encoding to strrpos' => [
+            "<?php mb_strrpos('subject', 'b', 2, 'utf-8');",
+            "<?php\n\nstrrpos('subject', 'b', 2);",
+        ];
+
+        yield 'It does not mutate mb_strrpos called via variable' => [
+            '<?php $a = "mb_strrpos"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForStrStr(): Generator
+    {
+        yield 'It converts mb_strstr to strstr' => [
+            "<?php mb_strstr('subject', 'b');",
+            "<?php\n\nstrstr('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_strstr is wrongly capitalize' => [
+            "<?php Mb_STRstr('subject', 'b');",
+            "<?php\n\nstrstr('subject', 'b');",
+        ];
+
+        yield 'It converts mb_strstr with part argument to strstr' => [
+            "<?php mb_strstr('subject', 'b', false);",
+            "<?php\n\nstrstr('subject', 'b', false);",
+        ];
+
+        yield 'It converts mb_strstr with encoding to strstr' => [
+            "<?php mb_strstr('subject', 'b', false, 'utf-8');",
+            "<?php\n\nstrstr('subject', 'b', false);",
+        ];
+
+        yield 'It does not mutate mb_strstr called via variable' => [
+            '<?php $a = "mb_strstr"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForStrToLower(): Generator
+    {
+        yield 'It converts mb_strtolower to strtolower' => [
+            "<?php mb_strtolower('test');",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It converts correctly when mb_strtolower is wrongly capitalize' => [
+            "<?php mB_StrTOloWer('test');",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It converts mb_strtolower with encoding to strtolower' => [
+            "<?php mb_strtolower('test', 'utf-8');",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It does not mutate mb_strtolower called via variable' => [
+            '<?php $a = "mb_strtolower"; $a("test");',
+        ];
+    }
+
+    private function provideMutationCasesForStrToUpper(): Generator
+    {
+        yield 'It converts mb_strtoupper to strtoupper' => [
+            "<?php mb_strtoupper('test');",
+            "<?php\n\nstrtoupper('test');",
+        ];
+
+        yield 'It converts correctly when mb_strtoupper is wrongly capitalize' => [
+            "<?php Mb_StrToupPer('test');",
+            "<?php\n\nstrtoupper('test');",
+        ];
+
+        yield 'It converts mb_strtoupper with encoding to strtoupper' => [
+            "<?php mb_strtoupper('test', 'utf-8');",
+            "<?php\n\nstrtoupper('test');",
+        ];
+
+        yield 'It does not mutate mb_strtoupper called via variable' => [
+            '<?php $a = "mb_strtoupper"; $a("test");',
+        ];
+    }
+
+    private function provideMutationCasesForSubStrCount(): Generator
+    {
+        yield 'It converts mb_substr_count to substr_count' => [
+            "<?php mb_substr_count('test', 't');",
+            "<?php\n\nsubstr_count('test', 't');",
+        ];
+
+        yield 'It converts correctly when mb_substr_count is wrongly capitalize' => [
+            "<?php MB_substr_COunt('test', 't');",
+            "<?php\n\nsubstr_count('test', 't');",
+        ];
+
+        yield 'It converts mb_substr_count with encoding to substr_count' => [
+            "<?php mb_substr_count('test', 't', 'utf-8');",
+            "<?php\n\nsubstr_count('test', 't');",
+        ];
+
+        yield 'It does not mutate mb_substr_count called via variable' => [
+            '<?php $a = "mb_substr_count"; $a("test", "t");',
+        ];
+    }
+
+    private function provideMutationCasesForSubStr(): Generator
+    {
+        yield 'It converts mb_substr to substr' => [
+            "<?php mb_substr('test', 2);",
+            "<?php\n\nsubstr('test', 2);",
+        ];
+
+        yield 'It converts correctly when mb_substr is wrongly capitalize' => [
+            "<?php mB_SuBsTr('test', 2);",
+            "<?php\n\nsubstr('test', 2);",
+        ];
+
+        yield 'It converts mb_substr with length argument to substr' => [
+            "<?php mb_substr('test', 2, 10);",
+            "<?php\n\nsubstr('test', 2, 10);",
+        ];
+
+        yield 'It converts mb_substr with encoding argument to substr' => [
+            "<?php mb_substr('test', 2, 10, 'utf-8');",
+            "<?php\n\nsubstr('test', 2, 10);",
+        ];
+
+        yield 'It does not mutate mb_substr called via variable' => [
+            '<?php $a = "mb_substr"; $a("test", 2, 10);',
+        ];
+    }
+
+    private function provideMutationCasesForStrRChr(): Generator
+    {
+        yield 'It converts mb_strrchr to strrchr' => [
+            "<?php mb_strrchr('subject', 'b');",
+            "<?php\n\nstrrchr('subject', 'b');",
+        ];
+
+        yield 'It converts correctly when mb_strrchr is wrongly capitalize' => [
+            "<?php MB_StRrcHr('subject', 'b');",
+            "<?php\n\nstrrchr('subject', 'b');",
+        ];
+
+        yield 'It converts mb_strrchr with part argument to strrchr' => [
+            "<?php mb_strrchr('subject', 'b', false);",
+            "<?php\n\nstrrchr('subject', 'b');",
+        ];
+
+        yield 'It converts mb_strrchr with encoding to strrchr' => [
+            "<?php mb_strrchr('subject', 'b', false, 'utf-8');",
+            "<?php\n\nstrrchr('subject', 'b');",
+        ];
+
+        yield 'It does not mutate mb_strrchr called via variable' => [
+            '<?php $a = "mb_strrchr"; $a("subject", "b");',
+        ];
+    }
+
+    private function provideMutationCasesForConvertCase(): Generator
+    {
+        yield 'It converts mb_convert_case with MB_CASE_UPPER mode to strtoupper' => [
+            "<?php mb_convert_case('test', MB_CASE_UPPER);",
+            "<?php\n\nstrtoupper('test');",
+        ];
+
+        yield 'It converts correctly when mb_parse_str is wrongly capitalize' => [
+            "<?php Mb_CoNvErT_Case('test', MB_CASE_UPPER);",
+            "<?php\n\nstrtoupper('test');",
+        ];
+
+        yield 'It converts mb_convert_case with MB_CASE_UPPER_SIMPLE mode to strtoupper' => [
+            "<?php mb_convert_case('test', MB_CASE_UPPER_SIMPLE);",
+            "<?php\n\nstrtoupper('test');",
+        ];
+
+        yield 'It converts mb_convert_case with constant similar MB_CASE_LOWER mode to strtolower' => [
+            "<?php mb_convert_case('test', E_ERROR);",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It converts mb_convert_case with numeric MB_CASE_LOWER (1) mode to strtolower' => [
+            "<?php mb_convert_case('test', 1);",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It converts mb_convert_case with MB_CASE_LOWER_SIMPLE mode to strtolower' => [
+            "<?php mb_convert_case('test', MB_CASE_LOWER);",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It converts mb_convert_case with MB_CASE_TITLE mode to ucwords' => [
+            "<?php mb_convert_case('test', MB_CASE_TITLE);",
+            "<?php\n\nucwords('test');",
+        ];
+
+        yield 'It converts mb_convert_case with MB_CASE_TITLE_SIMPLE mode to ucwords' => [
+            "<?php mb_convert_case('test', \MB_CASE_TITLE_SIMPLE);",
+            "<?php\n\nucwords('test');",
+        ];
+
+        yield 'It converts mb_convert_case with MB_CASE_FOLD mode to strtolower' => [
+            "<?php mb_convert_case('test', MB_CASE_FOLD);",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It converts mb_convert_case with MB_CASE_FOLD_SIMPLE mode to strtolower' => [
+            "<?php mb_convert_case('test', MB_CASE_FOLD_SIMPLE);",
+            "<?php\n\nstrtolower('test');",
+        ];
+
+        yield 'It does not convert mb_convert_case with mode as variable' => [
+            "<?php mb_convert_case('test', \$mode);",
+        ];
+
+        yield 'It does not convert mb_convert_case with missing mode argument' => [
+            "<?php mb_convert_case('test');",
+        ];
+
+        yield 'It does not mutate mb_convert_case called via variable' => [
+            '<?php $a = "mb_convert_case"; $a("test");',
+        ];
+    }
+
+    private static function defineMissingMbCaseConstants(): void
+    {
+        foreach ([
+            'MB_CASE_FOLD' => 3,
+            'MB_CASE_UPPER_SIMPLE' => 4,
+            'MB_CASE_LOWER_SIMPLE' => 5,
+            'MB_CASE_TITLE_SIMPLE' => 6,
+            'MB_CASE_FOLD_SIMPLE' => 7,
+        ] as $constantName => $constantValue) {
+            if (!\defined($constantName)) {
+                \define($constantName, $constantValue);
+            }
+        }
+    }
+}