As other answers suggest... Some guy (for whatever reason) decided that your old code should not work when you upgrade your PHP, because he knows better than you and don't care about what your code does or how simple it is for you to upgrade.
Well, if you can't upgrade your project overnight you can
downgrade your version of PHP to whatever version that worked
or...
use a shim (kind of polyfill) such as https://github.com/dshafik/php7-mysql-shim or https://github.com/dotpointer/mysql-shim, and then find a place for
include_once("choice_shim.php");
somewhere in your code
That will keep your old PHP code up and running until you are in a mood to update...