I have several live Drupal sites running on an Ubuntu server (10.04), Apache 2.2 with PHP 5.2.3-1. I recently started the work of setting up a test server that would be as close as possible to the live environment, but I stupidly didn’t check the PHP version that Ubuntu installed when I did an apt-get install php. I’ve now had my test server up for several weeks, and just realised that the version of PHP on my (edit: test) server is 5.3.2-1.
Part of the reason for the test server setup was to try a Drupal upgrade: 6.17 -> 6.19, and with it a bunch of modules, incidentally fixing errors along the way that were related to things that no longer silently fail in PHP5.3.
So the question is: will I regret not having the same version of PHP on my test server as on live? Instinct tells me that having different test and live environments is a Very Bad Idea, but perhaps it’s not such a big deal in the Drupal world (along the same lines of reasoning that not having version control is a Very Bad Idea, but that is a difficult task for CMS users). I really don’t want to mess with my (very much working) live environment if I don’t have to; is it a huge chore to downgrade the PHP version on my test environment?
Superuser should be able to tell you how to downgrade a package. I don’t think it is that difficult.
If you are planning to update your live server to 5.3 then it is a good idea to do it on the test server first. But in normal operations you will want your test and live servers to be as close as possible. Having 5.3 on one and 5.2 on another is a pretty big issue. From experience, you will regret it.