Error Identifier: pureFunction.parameterByRef
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);
/**
* @phpstan-pure
*/
function increment(int &$value): int // ERROR: Function increment() is marked as pure but parameter $value is passed by reference.
{
$value++;
return $value;
}
Why is it reported? #
A function marked as @phpstan-pure must not have side effects and must always return the same result for the same inputs. A parameter passed by reference (&$value) allows the function to modify the caller’s variable, which is a side effect. This contradicts the definition of a pure function.
Even if the function does not actually modify the referenced parameter, the mere presence of a by-reference parameter signals that the function’s contract allows mutation, which is incompatible with purity.
How to fix it #
Remove the by-reference parameter and return the computed value instead:
<?php declare(strict_types = 1);
/**
* @phpstan-pure
*/
-function increment(int &$value): int
+function increment(int $value): int
{
- $value++;
- return $value;
+ return $value + 1;
}
If the function needs to modify the caller’s variable, it is not pure. Remove the @phpstan-pure annotation:
<?php declare(strict_types = 1);
-/**
- * @phpstan-pure
- */
function increment(int &$value): int
{
$value++;
return $value;
}
How to ignore this error #
You can use the identifier pureFunction.parameterByRef to ignore this error using a comment:
// @phpstan-ignore pureFunction.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: pureFunction.parameterByRef