It's the sort of thing that was discussed in VIP about how to defend against content scrapers. Something in the PHP prevents the spiders from seeing the content.
If you have unique text on a HTML page, you only have to search for a part of it in double quotes and you can usually see where it is (and who has nicked it), if the page has been indexed. On PHP pages, spiders do not always do the job properly. Sometimes PHP site frames also prevent Adsense from running, in my experience. I really don't know enough about PHP to comment properly, I can only say what happens occasionally, and that it happens on PHP sites where I'm only able to put content on via a CMS, so I'm not in a position to mess around with it.