with the example they just gave, you need to call the webpage .php if it has php markups in it.
I use this menthod for includes on HTML pages it works the same but doesn't request php to be installed on other machine.
Server Side Include:
<!--include file="file.txt"-->
The file can be anything that you want to insert in there - HTML, TXT...
with this one you don't use php and you name the file either HTML or SHTML. It is recommended you name it .SHTML but it works with .HTMLon my cheapo geocities accnt =P
So you can do it php or html ssi.
php --> file.php
SSI HTML --> file.shtml