At some point in time, in the near future, there will be an Ubuntu update. You will think to yourself 'I'll update my server, it hasn't been done in a while' and you go straight into it, ignoring all of the warnings to back up your site, or sites. It may be foolish, you may be a gung-ho IT warrior but once you've finished and you're ready to restart, you keep those fingers crossed in trepidation.
Server coming up; it's all looking good; Apache has started; no errors; fantastic, now let's try those websites, because there's always more than one on the server!
It may not be as scary as trying to pull out of a spiral descent in a light aircraft but, like this life saving manoeuvre (1. reduce the power, 2. straighten the control yoke, 3. straighten the rudders), there's three easy steps to follow:
1. re-install PHP (remove and install)
2. ensure the database(s) privileges are reset to user again ('grant all privileges' and 'flush privileges')
3. re-start mysql and apache servers
Trust me...it works!