Error Identifier: purePropertyHook.parameterByRef

← Back to purePropertyHook.*

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);

final class Foo
{
	private int $backing = 1;

	public int $value {
		/** @phpstan-pure */
		set(int &$value) {
			$this->backing = $value;
		}
	}
}

Why is it reported? #

A property hook (PHP 8.4+) marked as @phpstan-pure must not have side effects and must always behave the same for the same inputs. A parameter passed by reference (&$value) lets the hook modify the caller’s variable, which is a side effect and contradicts purity.

Note that PHP itself does not allow by-reference parameters in property hooks — declaring one produces a fatal error (Parameter $value of set hook must not be pass-by-reference). This identifier exists for consistency with pureMethod.parameterByRef and pureFunction.parameterByRef, which apply to regular methods and functions where by-reference parameters are permitted.

How to fix it #

Do not pass the hook parameter by reference. A set hook receives the assigned value directly:

 public int $value {
 	/** @phpstan-pure */
-	set(int &$value) {
+	set {
 		$this->backing = $value;
 	}
 }

Because a set hook mutates state, it is inherently impure; remove the @phpstan-pure annotation as well:

 public int $value {
-	/** @phpstan-pure */
 	set {
 		$this->backing = $value;
 	}
 }

How to ignore this error #

You can use the identifier purePropertyHook.parameterByRef to ignore this error using a comment:

// @phpstan-ignore purePropertyHook.parameterByRef
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: purePropertyHook.parameterByRef

Rules that report this error #

• PHPStan\Rules\Pure\PureFunctionRule [1]
• PHPStan\Rules\Pure\PureMethodRule [1]
• PHPStan\Rules\Pure\PurePropertyHookRule [1]