Chat Room not showing up

I am getting the following message instead of the chat room page fpr dating page, hopefully someone can help. ( I hve left out the full address links of the folders for privacy.

Fatal error: Uncaught Error: Call to undefined function mysql_escape_string() in
/wp-content/themes/DA/tpl-chatroom.php:33 Stack trace: #0
/wp-includes/template-loader.php(75): include() #1
/wp-blog-header.php(19): require_once(‘/homepages/37/d…’) #2 /index.php(17): require(‘/homepages/37/d…’) #3 {main} thrown in
/wp-content/themes/DA/tpl-chatroom.php on line 33

Any ideas?