Someone

function get_array_key($array, $n){
	$i = 0;
	foreach ($array as $key => $value){
		if ($i = $n) {
			return $key;
			break;
		}
		$i++;
	}
	return null;
}

or

function get_array_key($array,$n=0){
	reset($array);
	$x = each($array);
	return $x[$n];
}