Snow Leopard updated me to PHP 5.3 ok but it didn’t leave me with a working php.ini. Just the php.ini.default.
The main upshot of this was broken date and thus broken header errors all over the shop. Rename the deafult to just php.ini (in /private/etc/ usually) and got into it uncomment the date.timezone = Â and set it to your timezone. Will look something like this
; Defines the default timezone used by the date functions
date.timezone = Europe/London