After upgrading your app to PHP 7+, there is a very slim chance you will receive the following error:
Fatal error: Uncaught Error: Call to undefined function mysql_connect()
This is due to the removal of the mysql_connect function from PHP 7+.
If you receive that error, it means some of your app's code, such as a WordPress plugin or theme, is not compatible with PHP 7+.
To resolve this error, simple switch the app to PHP 5.6 until the code is updated to work with PHP 7+.
First, open your app's Settings tab in ServerPilot.
Then, click the blue pencil next to your current PHP version.
Select PHP 5.6 from the drop-down list and click Update.