You should go back to a version of PHP before 7 for any existing sites. Hosting companies will continue to support earlier versions for the next decade or so, you just can't upgrade above PHP 6 for those sites.
You can use our MySQLi Server Behaviors for new sites using PHP 7. MySQL has been replaced by MySQLi (the "i" stands for improved). If you build sites with those they will continue to work for the foreseeable future on future versions of php.