I upgraded my Ubuntu 10.04 to Linux Mint 14. After installing LAMP I got a Drupal WSOD on a previously well working site. As it turned out most errors came from deprecated php-functions and deprecated call by references to functions, introduced with the release of PHP 5.4.
At first I was a somewhat encouraged to solve those deprecated functions, but I gave up pretty soon. Drupal 6 isn’t designed for 5.4. Tweaking Drupal feels the same as upgrading to Drupal 7. Therefor, I keep it to PHP 5.3.
To downgrade 5.4 to 5.3 I recommend this script on the Ubuntu forums.