Upgrading to Snow Leopard installs a new version of php.(5.3.0)
The php.ini file, which should be at /etc/php.ini, will need to be created.
You do this by editing /etc/php.ini.default, and renaming it to php.ini.
The changes required are
1) add the correct time zone,
; Defines the default timezone used by the date functions
2) change the three references of the mysql.sock address from
Then restart apache by stopping and restarting
System Preferences>Sharing>Web Sharing
More detailed instructions are at
Install Apache/PHP/MySQL on Snow Leopard