method.impure

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;
 	}
 }