My PHP version is below 7.0, but I still get this error. What could be the problem?

If you're on a PHP version that *should* support `mysql_connect()`, the MySQL extension might not be enabled in your `php.ini` file. Check your `php.ini` file for the line `extension=mysql`. If it's commented out (starts with a semicolon), uncomment it and restart your web server (Apache, Nginx, etc.).