Possible work around for running PHP as CGI (or ISAPI) on Windows 2003 SP1 64bit.

Source:

Installing PHP 4 or 5 on Windows Server 2003 x64 results in http error 505. This is not a PHP bug. It is due to running a 32-bit ISAPI DLL under the 64-bit version of IIS 6, as documented here: http://support.microsoft.com/?id=895976

Solution: Flip the Metabase flag for Enable32bitAppOnWin64 as described in http://support.microsoft.com/?id=894435 ,
like this: CSCRIPT ADSUTIL.VBS SET W3SVC/AppPools/Enable32bitAppOnWin64 1

(Alternatively with full path: cscript %SYSTEMDRIVE%\inetpub\adminscripts\adsutil.vbs SET W3SVC/AppPools/Enable32bitAppOnWin64 1)