This is more a bookmark for my reference than anything, but if you have a stock CentOS 5.x installation (maybe from a VPS image or a clean install on a dedicated box) you're going to have issues with Drupal 6.x.

It comes down to PHP version. It ships with PHP 5.1.6 and Drupal these days pretty much depends on PHP 5.2.x upwards to work. Utilities like Drush do too, so leaving PHP as it is, you'll find, is not really an option. To confirm you will have this problem, execute this command to check your PHP version: