Update. There are potential issues when upgrading to 5.3. Surprise, surprise.
5.3.8 is not their default and that includes PHP help thingys in cpanel. Stay far away from "PHP Quick-Config" (can be disabled). So before going 5.3 delete php.ini (or better save it) and .htaccess also needs a little change. Quick-Config work directly with php.ini for version 5.2 so useless, perhaps able to mess up even. There are entries in exisiting 5.2 focused php.ini that hard links to folders with "extensions". These are specific to php version. Upgrading only upgrade PHP itself not surrounding stuff.
Fully documented if you look for correct info. Cool but if in doubt contact their support. They say "YOU" are supposed to build new php.ini (98% similar to the one for 5.2) but if problems they will give you one. I wont link as certain things are being debated internally in my head, almost real time Just go to their forum and FAQ if not sure.
Other weird finding is that W3 Total Cache seems to be almost incompatible with Hostgator, "compatibility" check reveal features as common as mod_expires are missing and required for this and that. Well ignore and do before/after test using these server features (browser cache), they are available and do work. But since W3 miss detecting other tools might as well. Running phpinfo does not really show much. They must hide certain Apache features. I also tried to SSH in to server but cant find much. SSH is "limited" so probably why. They have a FAQ page which links all paths to relevant libraries/scripts etc. so have fun searching.
No details is crap so check difference below in php.ini, old vs. new Enough to fire up Google I bet.
Very important of course but 99 out of 100 settings are not. With no entries defaults for 5.3 is used so custom php.ini is like custom CSS for Catalyst. Only settings different than defaults are required.
;extension_dir = "/usr/local/lib/php/extensions/no-debug-non-zts-20060613"
extension_dir = "/opt/php53/lib/php/extensions/no-debug-non-zts-20090626"