I upgraded my server from Ubuntu 12.04 to 14.04 this weekend. It took forfricking ever, thanks to one little file that got added to my Apache configuration files. I never found anything about it online, either; the only way out was through prolonged cussing.
So (search engines!), if you had a working Ubuntu 12.04 server that you upgraded to 14.04, only to discover that your PHP files are no longer processed by PHP (i.e. your server just spits out the PHP code as the page): check if /etc/apache2/conf-available/php5-cgi.conf exists. If so, try disabling it in the commandline (a2dissite php5-cgi.conf) and restarting Apache.
Otherwise there were a mess of issues, but they were pretty easy to address: find a service that’s broken, check the error log, fix the error, repeat until the service works. It’s pretty much the sysadmin equivalent of Shinji’s “Target in the center; pull the switch. Target in the center; pull the switch.” training in Evangelion.
I am genuinely considering just spinning up a new VPS and forklifting email and the websites over, rather than attempting an in-place upgrade. Ugh.
That was Plan B, if my first attempt failed (I did, of course, make a snapshot of my working server prior to updating). The in-place upgrade would have been markedly faster than starting fresh, for me, if you ignore that one bloody file… though I’m not convinced that it wouldn’t have done the same thing to me if I had started from scratch.
Plan C was to say “eff it” and wait for 16.04.