In case, you are using PHP7 already, the formerly deprecated functions
mysql_* were removed entirely, so you should update your code using the
PDO-functions or mysqli_* functions instead.

If that's not possible, as a workaround, rubo77 created a small PHP include
file, that recreates the old mysql_* functions with mysqli_*()-functions:
https://github.com/rubo77/php-mysql-fix