One thing changed with Leopard is the socket for Mysql. It moved to /private/tmp, so you may need to configure your php.ini file to point it to the new location.
To do so, open the file ‘/private/etc/php.ini‘, (if no such file exists, then make a copy of ‘/private/etc/php.ini.default‘ naming it ‘php.ini‘) and edit that.
You have two lines to modify:
mysql.default_socket =
becomes:
mysql.default_socket = /private/tmp/mysql.sock
and mysqli.default_socket =
becomes:
mysqli.default_socket = /private/tmp/mysql.sock
of course, from the sharing pref pane, stop the server and restart it and voila!
