Website down after server upgrade to PHP 5.3

The host we use has kindly upgraded their PHP from 5.2 to 5.3. Some sites built with the most recent versions of MODX Revolution (2.2) are unaffected. Sites built earlier in 2011 with earlier versions of Modx Revolution are now down, mostly showing no content at all and I can't log into the manager area.

Tried uploading MODX Revo 2.2 zip file, extracting it in the CPanel file manager, running /setup to upgrade the installation. After choosing the option to upgrade (rather than new install) the setup process stops and the screen flashes "500 Error - site temporarily unavailable". Then a check at the front end of the site reveals the error code:

Fatal error: Class 'xPDODriver_' not found in /home/judewalk/public_html/core/xpdo/xpdo.class.php on line 1822

I would be massively grateful for any advice about how to get these sites back up and working again.

Some progress made. Noticed something odd that was stopping me upgrade the installation: backticks had been added inside the single quotes in the config.inc.php file, plus there was no characterset declared, plus the following lines were missing :

$database_dsn = 'mysql:host=localhost;dbname=judewalk_modx;charset=utf8';
$config_options = array (
);
$driver_options = array (
);

After adding those lines, plus the characterset, and removing the backticks, I was able to complete the upgrade, then log in with the old login details, plus the front end of the site is working. Great!!

Sometimes I am amazed by what someone who has absolutely no idea what they are doing can achieve.