Php random string generator

   public function generateRandomString() {
        $length = 20;
        $characters = "0123456789abcdefghijklmnopqrstuvwxyz";
        $string = "";   

        for ($p = 0; $p < $length; $p++) {
            $string .= $characters[mt_rand(0, strlen($characters)-1)];
        }

        return $string;
    }