This is just a shot in the dark as I don't have experience of php with maemo, but I've used it elsewhere. I assume you're trying to run the program in command line. If you're trying to view the page with web browser, this won't help you.

First you have to give the file execute privileges by issuing:

chmod u+x file.php

and after that you can run

./file.php

assuming you're in the same directory as the file.

If it doesn't work, you should see if php is installed correctly by writing

which php

You should get something like /bin/php
Add to the beginning of the php-file:

#!/bin/php

or whatever it is you get from the previous command. I don't know how it goes with N800, but at least this is the way it works in linux generally.