I was having som weird issues with performance on my PowerBook under Mac OS X 10.4.3, so I decided to upgrade to 10.4.5.
After my machine rebooted I could no longer connect to my MySQL databases through PHP. A quick web search turned up this MacRumors forum post. I just had to change a line in my /etc/php.ini file from this:
mysql.default_socket =
to this:
mysql.default_socket = /tmp/mysql.sock
I restarted and everything was peachy.