I am attempting to create new items in this context menu.

Some file extensions cannot be added. I have tried both the TweakUI method and the manual registry editing method but both seem to yield nothing (no item appears in the menu) for particular file types (.php, in this case).
Ultimately I want to be able to select a .PHP file (could be blank, or a template) from the New menu and have it spawned where the context menu is brought up. The Template section of TweakUI could do this, if it were not for the fact that .php files refuse to work.
Any ideas why I am unable to add a .php item to the 'New' menu?