Problem with: "Warning: mysql_real_escape_string()"

Hello out there,
I get these warnings in my Mystile theme. I use WordPress 4.1 and PHP 5.6:

“Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: No such file or directory in /var/www/tiesquare.dk/public_html/wp-content/themes/mystile/functions/admin-interface.php on line 111”

and

“Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: A link to the server could not be established in /var/www/tiesquare.dk/public_html/wp-content/themes/mystile/functions/admin-interface.php on line 111”

If you go to line 111 in admin-interface.php you will find this code:

$_page = mysql_real_escape_string( strtolower( trim( strip_tags( $_REQUEST['page'] ) ) ) );

Here a link for my website: tiesquare.dk
I would be really happy if someone would help me with this problem.

Kind regards,
Joha5033 ??