Error Identifier: method.impure

← Back to method.*

Every error reported by PHPStan has an error identifier. Here’s a list of all error identifiers. In PHPStan Pro you can see the error identifier next to each error and filter errors by their identifiers.

Code example #

<?php declare(strict_types = 1);

class Foo
{
	/** @phpstan-pure */
	public function calculate(): int
	{
		return 1;
	}
}

class Bar extends Foo
{
	/** @phpstan-impure */
	public function calculate(): int // ERROR: Impure method Bar::calculate() overrides pure method Foo::calculate().
	{
		return random_int(0, 100);
	}
}

Why is it reported? #

When a parent class or interface declares a method as pure (@phpstan-pure), all overriding methods must also be pure. A pure method guarantees it has no side effects and its return value depends only on its arguments. Code that calls the parent method relies on this guarantee — if a child class introduces side effects or non-deterministic behavior, it violates the contract.

How to fix it #

Remove the @phpstan-impure annotation if the method is actually pure:

 class Bar extends Foo
 {
-	/** @phpstan-impure */
+	/** @phpstan-pure */
 	public function calculate(): int
 	{
-		return random_int(0, 100);
+		return 42;
 	}
 }

If the method genuinely needs to be impure, reconsider the parent’s purity contract. Remove @phpstan-pure from the parent method if purity cannot be guaranteed across all implementations:

 class Foo
 {
-	/** @phpstan-pure */
 	public function calculate(): int
 	{
 		return 1;
 	}
 }

If the parent class uses @phpstan-all-methods-pure, override the specific method in the parent instead:

+/** @phpstan-all-methods-pure */
 class Foo
 {
+	/** @phpstan-impure */
 	public function calculate(): int
 	{
 		return 1;
 	}
 }

How to ignore this error #

You can use the identifier method.impure to ignore this error using a comment:

// @phpstan-ignore method.impure
codeThatProducesTheError();

You can also use only the identifier key to ignore all errors of the same type in your configuration file in the ignoreErrors parameter:

parameters:
	ignoreErrors:
		-
			identifier: method.impure

Rules that report this error #

• PHPStan\Rules\Methods\MethodSignatureRule [1]
• PHPStan\Rules\Methods\OverridingMethodRule [1]