Deprecated: mysql_connect(): The mysql extension is deprecated and wil

Boa noite,

Alguém poderia mim ajudar a resolver esse erro:

Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in /home/u129091882/public_html/session_cidade.php on line 8

Fatal error: Call to undefined function session_is_registered() in /home/u129091882/public_html/index.php on line 8

Desde já agradeço.