Added ImplodeFunctionRule

src/Type/Php/ImplodeFunctionReturnTypeExtension.php

@@ -8,8 +8,10 @@
 use PHPStan\Type\Accessory\AccessoryLiteralStringType;
 use PHPStan\Type\Accessory\AccessoryNonEmptyStringType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
+use PHPStan\Type\ErrorType;
 use PHPStan\Type\IntersectionType;
 use PHPStan\Type\StringType;
+use PHPStan\Type\UnionType;
 
 class ImplodeFunctionReturnTypeExtension implements DynamicFunctionReturnTypeExtension
 {
@@ -62,6 +64,17 @@ public function getTypeFromFunctionCall(
 			}
 		}
 
+		if ($arrayType->getIterableValueType()->isArray()->yes()) {
+				return new ErrorType();
+		}
+		if ($arrayType->getIterableValueType() instanceof UnionType) {
+			foreach ($arrayType->getIterableValueType()->getTypes() as $subType) {
+				if ($subType->isArray()->yes()) {
+					return new ErrorType();
+				}
+			}
+		}
+
 		if ($arrayType->getIterableValueType()->isLiteralString()->yes() && $separatorType->isLiteralString()->yes()) {
 			$accessoryTypes[] = new AccessoryLiteralStringType();
 		}

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -510,6 +510,8 @@ public function dataFileAsserts(): iterable
 
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/eval-implicit-throw.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-5628.php');
+
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/implode.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/implode.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace ImplodeErrors;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	public function invalidArgs(): void
+	{
+		assertType('*ERROR*', implode('',['12','123',['1234','12345']]));
+		assertType('*ERROR*', implode('',[['1234','12345']]));
+	}
+}