For reference I had this issue with joomla and it took me a day to sort out.
"Database connection error (1): The MySQL adapter 'mysql' is not available. Database connection error (1): The MySQL adapter 'mysql' is not available."
The problems started when I moved a copy of the joomla site from our staging server to our live server both of the servers have plesk 12.5 and I imported full Plesk backup in order to update the live server.
On importing the Plesk backup this caused the error as a different version of PHP was selected on the live server. I then proceeded to waste a whole day trying different versions of PHP and checking which mysql handlers were working. Eventually I notice that the php.ini file did not match another Joomla site on the same server resetting version in plesk PHP fixed that problem. I had versions Joomla Akeeba backups to use so I deleted the domain and reinstall from scratch using the akeeba backup. This got the site working and I then reinstated my nginx settings and the problem recurred.
I was using nginx and I had the proxy_cache enabled so what was happening was that once the error had occurred I could never get past the error page because nginx had cached it. Effectively every time I made the change and got errors the error pages were cached by nginx and then served for future requests. Here are the additional nginx settings that caused the problem.
proxy_cache qms;
proxy_cache_valid 200 1d;
All very annoying.
Paul