Error Identifier: pureFunction.parameterPassedNotByRef

← Back to pureFunction.*

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);

/**
 * @pure-unless-parameter-passed $flag
 */
function format(string $subject, bool $flag = false): string // ERROR: Function format() is marked @pure-unless-parameter-passed for parameter $flag, but $flag is not passed by reference.
{
	return $subject;
}

Why is it reported? #

The @pure-unless-parameter-passed tag marks a function as pure unless an argument is passed for the named parameter. It is meant for optional by-reference out-parameters, like the $count parameter of str_replace(): writing into the caller’s variable through such a parameter is the side effect that makes the call impure.

A parameter passed by value cannot produce a side effect just by being passed — the function only receives a copy. If the function had side effects when $flag is passed, they would have to come from its body, and those side effects are not conditional on the argument in a way that can be described or verified. The tag therefore has no meaning on a by-value parameter.

How to fix it #

If the function has no side effects, mark it as pure instead:

 /**
- * @pure-unless-parameter-passed $flag
+ * @phpstan-pure
  */
 function format(string $subject, bool $flag = false): string

If the parameter is meant to be an out-parameter, declare it by reference and describe the written type with @param-out:

 /**
+ * @param-out int $count
- * @pure-unless-parameter-passed $flag
+ * @pure-unless-parameter-passed $count
  */
-function format(string $subject, bool $flag = false): string
+function format(string $subject, int &$count = 0): string
 {
+	$count = 1;
+
 	return $subject;
 }

How to ignore this error #

You can use the identifier pureFunction.parameterPassedNotByRef to ignore this error using a comment:

// @phpstan-ignore pureFunction.parameterPassedNotByRef
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: pureFunction.parameterPassedNotByRef

Rules that report this error #

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