Any further thoughts on this? I'm not sure where else to explore.
I've seen this asked a few times in the mediawiki-l archives, so just go over what I've fiddled with:
* I've confirmed that the MySQL PHP extension exists.
* I've turned on debugging, and only received a single header dump within debug file. I can provide it upon request, but there doesn't look to be anything interesting in there.
* The <html><body></body></html> thing I'm seeing is when Mozilla receives no data from the service. That literal output has nothing to do with PHP - my apologies.
* Adding display_errors at various points in various files doesn't help at all (but, as confirmation that php error logging IS working, if I make a syntax error in my meanderings with MW, that is logged successfully).
* Attempting URLs like index.php?title=Special:Specialpages gave me the same browser behavior, and besides the expected GET change, the same header output in the debug log.