After installing Leopard, the previous Apache configuration file in /etc/httpd/httpd.conf is no longer used. The Apache configuration sits in /etc/apache2/httpd.conf now.
In order to enable PHP again, I uncommented the following line first:
LoadModule php5_module libexec/apache2/libphp5.so
Second, to get my virtual hosts running again, I used the adapted Leopard version of Patrick Gibsons excellent utility
virtualhost.sh which even automatically moves all prior (Tiger) virtual hosts.
Finally to get rid of the error
Can't connect to local MySQL server through socket '/var/mysql/'
I successfully followed the steps found here:
sudo mkdir /var/mysql/
sudo ln -s /tmp/mysql.sock /var/mysql/mysql.sock


Everything is up and running fine now.