Note: this only applies to users who’re running alternate PHP versions in Sympl 12 and 13, those running earlier versions won’t be affected by this.
Anyone affected by this should be able to stop any manually started PHP processes, then:
sudo sympl upgrade
sudo sympl-php-configure
That should fix things up and sites should start working again.
Note that you may want to run sudo apt update ; sudo apt install sympl-web to check for any ‘broken’ packages hanging over from the problem PHP configurations.
For anyone who’s affected but only just noticed, this will get automatically installed overnight and should fix itself when it does.