static public function callback($array, $callback)
	{
	    if (is_array($array)) foreach ($array as $k=>$v) {
	        if (is_array($v)) {
	            $array[$k] = static::callback($v, $callback);
	        } else {
	            $array[$k] = $callback($v);
	        }
	    }
	    return $array;
	}