<?php

class Pituh implements ArrayAccess {
    private $data;
    public function offsetExists ($offset) {
        return isset($this->data[serialize($offset)]);
    }
    public function offsetGet ($offset) {
        return $this->data[serialize($offset)];
    }
    public function offsetSet ($offset, $value) {
        $this->data[serialize($offset)] = $value;
    }
    public function offsetUnset ($offset) {
        unset($this->data[serialize($offset)]); 
    }
}

$a = new Pituh;

$a[['какой']] = 'багор';

echo $a[['какой']];