As said above, using mod rewrites in htaccess can disguise a php site as anything else BUT php. First thing I would try is to view source and see what kinds of extensions are on nav links. are they html, asp, php, etc? If that doesn't lead you anywhere, try looking at comments in the code or in the stylesheets (css) or javascript (js) files for clues. Most sites out there are built using a cms like wordpress or drupal or something like that. If this is the case, chances are there is something left in the comments somewhere in the code of what kind of software being used and you can google it to find it.
For example, just looking at the html tag of this site shows a popular forum software name called vbulletin. The vbulletin wiki says it uses php and mysql. Not to say that this is definitely the language and db uses, but it's pretty likely. You just gotta look for clues that the developer may have left behind.
But I guess the real question is...why do you want to know?