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

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"

Reproduce the PHP 8.1.8 example with array_walk and array_walk_recursive, then inspect the existing documentation for readonly properties and these array functions. Done means the relevant documentation explains why the read-only array triggers the error during walking, with an example or cross-reference covering both functions.