As of 5.3.15 (if you are still on 5.3 branch) you have to add extension=php_com_dotnet.dll line into your php.ini to have COM, DOTNET, VARIANT and similar classes available and working.
From PHP Change log:
Fixed bug #62146 com_dotnet cannot be built shared
Because of this, the official PHP builds for Windows are now built with "--enable-com-dotnet=shared" option, which means no COM/DOTNET support by default.
Some of my projects that I currently maintain use adodb library which uses VARIANT class and fails to work without it.