array_walk and (_recursive) cause an error if applied to readonly properties

[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
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"

[cmb69]

Yeah, that error is raised when fetching the property for write context (since it's a by-ref parameter):

https://github.com/php/php-src/blob/395b6a9674cfa70c97687dd49b97e34372600f02/Zend/zend_execute.c#L3096-L3105

We likely could treat arrays like objects, but that might even be more confusing. In my opinion, that's a documentation issue.

[SpiGAndromeda]

Ok I see. What is the way to propose changes to the PHP documentation?

[cmb69]

I suggest to wait a while; maybe others see that differently. If not, this ticket can be transferred to php/doc-en, and we can go from there.

[iluuu1994]

array_walk_recursive needs to accept by reference so that by reference modifications are possible at all, even if you don't use it. In PHP it's not obvious that $this->variables is passed by reference as the passing semantics are controller by the callee and not the caller. https://wiki.php.net/rfc/explicit_send_by_ref would make the behavior more obvious but not solve the problem, as we'd likely want to throw if you pass something that should be passed by reference by value (at least once we remove the BC layer).

A simple workaround would be:

$variables = $this->variables;
array_walk_recursive($variables, static function(mixed $variable) use (&$objectVariables) {

[SpiGAndromeda]

@iluuu1994 I just didn't use constructor promotion as a workaround. So array is checked before written as a readonly property. But I guess it should still be documented. :)