Random | Unique string generate in PHP

function random_string($length) {
		$key = '';
		$keys = array_merge(range(0, 9), range('a', 'z'), range('A', 'Z'));

		for ($i = 0; $i < $length; $i++) {
			$key .= $keys[array_rand($keys)];
		}
		return $key;
	}