<?php
class A implements ArrayAccess {
    public function &offsetGet($n): mixed {
    	return null;
    }
    public function offsetSet($n, $v): void {
    }
    public function offsetUnset($n): void {
    }
    public function offsetExists($n): bool {
    	return false;
    }
}

set_error_handler(function () {
    $GLOBALS['a'] = null;
});

$a = new A;
$a[$c] = 'x' ;
var_dump($a);

$a = new A;
$a[$c] .= 'x' ;
var_dump($a);

$a = new A;
$a[$c][$c] = 'x' ;
var_dump($a);
?>