I have ran aptitude install php5-mysql (and restarted MySQL/Apache 2), but I am still getting this error:
aptitude install php5-mysql
Fatal error: Call to undefined
Well, this is your chance! It looks like PDO is ready; use that instead.
Try checking to see if the PHP MySQL extension module is being loaded:
If it's not there, add the following to the php.ini file:
php.ini
extension=php_mysql.dll