Try the basic demo:

TT Red Gadgets
TT Blue Gadgets

You need to uncomment the QUERY_STRING code in 'content-freshener.php' for the next two links to work.

A Lovely Red Lorry
A dinky blue meanie

This is an PHP file. It is executable i.e. the server considers it a script.

If you look at the source code of this file, you'll see the 'content-freshener.php' call below. This instance is being called within the HTML output of this file, so it's bracketed by PHP code. (To see an instance where it's being called within the PHP code itself, look at the top of this file. There, it's been commented-out.)