rocknbil - 2:11 am on Nov 4, 2009 (gmt 0)

I'm sorry, where are my manners. LOL That search will likely bring you the web sites of every PHP coder on the 'net.

Start here [w3schools.com], Simple and short tutorials, they discuss PHP includes there.
The whole PHP manual [php.net]

FYI, there are many things you can do with SSI besides simple includes: tutorial [httpd.apache.org] and full manual [httpd.apache.org].