possiblyImpure.superglobal

Code example

<?php declare(strict_types = 1);

/**
 * @phpstan-pure
 */
function getUserAgent(): string
{
	return $_SERVER['HTTP_USER_AGENT'] ?? 'unknown';
}

Why is it reported?

This identifier is not reported in practice because superglobal access is always considered definitely impure, not just possibly impure. PHPStan reports impure.superglobal instead.

The function or method is marked as @phpstan-pure but accesses a superglobal variable such as $_GET, $_POST, $_SERVER, $_SESSION, $_COOKIE, $_FILES, $_ENV, or $_REQUEST. Superglobals represent external mutable state that can change between calls, so reading from or writing to them is a side effect. Pure functions must always return the same result for the same inputs.

How to fix it

Pass the needed value as a parameter instead of accessing the superglobal directly:

 /**
  * @phpstan-pure
  */
-function getUserAgent(): string
+function getUserAgent(string $userAgent): string
 {
-	return $_SERVER['HTTP_USER_AGENT'] ?? 'unknown';
+	return $userAgent;
 }

Alternatively, remove the @phpstan-pure annotation if the function intentionally relies on superglobal state.