PHP serves blank file when loading from UserDir

[j snod <php@xxxxxxxxxxxxxxxx>]

Hi.  I have run into a problem where PHP serves a blank file when 
accessing it via an Apache UserDir.  PHP works fine otherwise.  Eg:

http://www.server.com/file.php WORKS FINE

http://123.456.789.123/~username/file.php DOES NOT

I'm fairly sure it's not an Apache issue because accessing HTML files 
via a UserDir works fine, eg:
http://123.456.789.123/~username/file.html

I know that PHP is loaded because when I access a non-existent file and 
trigger a 404, I get the following in my ServerSignature:

Apache/2.2.2 (Fedora) DAV/2 PHP/5.1.6 mod_python/3.2.8 Python/2.4.3 
mod_ssl/2.2.2 OpenSSL/0.9.8a SVN/1.3.2 Server at server.com Port 80

Everything else about this configuration works fine except for this 
UserDir problem.  I have a similar configuration, but with PHP4, and it 
works as expected.  Is this a change with PHP5?  A potential bug?  There 
are no error messages anywhere (Apache or PHP logs), only a blank page. 
  I've completely exhausted my Google-fu and have failed.  I know other 
people have experienced this, but have yet to find a solution.  Help me 
Obi Wan, you're my only hope!

Thanks,
-j