how to replace Replace mcrypt_encrypt?

with php 7 my website does not work anymore correctly. i tried the reccomendation on the website: https://gist.github.com/odan/c1dc2798ef9cedb9fedd09cdfe6e8e76, but it still does not work. can anyone help me?

errore message:

Fatal error: Uncaught Error: Call to undefined function mcrypt_encrypt() in /is/htdocs/wp12891720_A49KPJUSEX/www/chat.php:115 Stack trace: #0 /is/htdocs/wp12891720_A49KPJUSEX/www/chat.php(131): Crypt::encode('5c46ea88788a0aa...', Array) #1 {main} thrown in /is/htdocs/wp12891720_A49KPJUSEX/www/chat.php on line 115

class Crypt {
            static public function encode($key, $password) {
              $key .= date('Y-m-d H');
              return base64_encode(mcrypt_encrypt(MCRYPT_RIJNDAEL_256, md5($key), json_encode($password), MCRYPT_MODE_CBC, md5(md5($key))));
            }
          }