Source - http://press.redhat.com/2011/01/13/red-hat-enterprise-linux-5-6-now-avai...
Just as a note to everyone, the recently shipped RHEL 5.6 breaks a usual release pattern of not upgrading software packages across major versions within a release. Now, PHP 5.3 will be included once your systems are updated to RHEL 5.6. This was done to ensure applications have little difficulty when upgrading to RHEL 6 this year.
This is just another reminder that we need to be diligent about making sure module are 5.3-compliant. You can find PHP 5.3-related issues here - https://drupal.org/project/issues/search/?issue_tags=PHP%205.3
