[php-src] Issue #9117: array_walk and (_recursive) cause an error if applied to readonly properties

Issue: https://github.com/php/php-src/issues/9117 Author: SpiGAndromeda ### Description The following code should check if the passed array of variables contains any elements that are objects. If that is so, an exception is thrown: ```php <?php declare(strict_types=1); final class Variables { /** * @param array<string, mixed> $variables */ public function __construct(private readonly array $variables) { $objectVariables = []; array_walk_recursive($this->variables, static function(mixed $variable) use (&$objectVariables) { if (is_object($variable)) { $objectVariables[] = $variable; } }); if (0 !== count($objectVariables)) { throw VariablesContainObjectsException::create(); } } /** * @return array<string, mixed> */ public function get(): array { return $this->variables; } } ``` An Error is thrown at the 'array_walk_recursive' call: Cannot modify readonly property Variables::$variables. The trace provides no additional information. (I also tested it with 'array_walk' with the same result) As far as I understood, walking through an array without modifiying anything should not cause an error, because no value is manipulated. If that behavior is correct, it should be documented somewhere. I found no information about such behavior of readonly properties in array methods. ### PHP Version PHP 8.1.8 ### Operating System PHP 8.1 docker image "php:8.1-apache"