possiblyImpure.propertyUnset

Code example

<?php declare(strict_types = 1);

class Foo
{
	public ?string $name = null;

	/** @phpstan-pure */
	public function reset(): void
	{
		unset($this->name);
	}
}

Why is it reported?

This identifier is not reported in practice because property unset is always considered definitely impure, not just possibly impure. PHPStan reports impure.propertyUnset instead.

The function or method is marked as @phpstan-pure, meaning it must not cause side effects. Unsetting a property modifies object state, which is a side effect. Since the object might be referenced elsewhere, this mutation is possibly impure.

How to fix it

Remove the @phpstan-pure annotation if the method intentionally modifies state:

-/** @phpstan-pure */
 public function reset(): void
 {
 	unset($this->name);
 }

Or restructure the code to avoid mutating properties in a pure context:

 /** @phpstan-pure */
-public function reset(): void
-{
-	unset($this->name);
-}
+public function withoutName(): static
+{
+	$clone = clone $this;
+	$clone->name = null;
+	return $clone;
+}