Huh? Do you want to include the PHP page's source in the file, or do you want it to be processed? Either way, your \"XHTML Document\" will have to be a PHP page that outputs XHTML (well, you could use an IFRAME or LAYER to include the PHP page's contents).
Well, because its the portal for a forum software - the original php is outputting xhtml - but that side of it is complex as hell and I don't want to touch it, where the xhtml templates are easy to access and edit and I can understand essentially what it is doing.
The Iframe works nicely, don't know why you don't care for it. (www.midnightsquadron.com is what I'm fiddling with)
I don't like IFRAMEs because they were originally an IE-specific addition and therefore there's nothing binding Firefox, Opera, et al to even implement it (in fact, Netscape/Mozilla has LAYER for the same purpose). Luckily, since it's been around since around the days of HTML 3.0, most browsers do implement it.