I wonder how useful it would be to have PHP parser be able to define custom tags for HTML and be able to bind various functions (or classes) to them. I’m sure somebody must be doing something like that among dozens of frameworks and template systems. However, to do it in a really nice, fast and clean way would require support from the engine parser. It might be nice as a kind of shortcut template system. It might also make much easier to use PHP with various visual design tools.
On the other hand, ColdFusion does that, and ASP does that, and not everybody likes it. But ColdFusion takes it to the extreme – language is custom tags there (I’m not a CF programmer and never played one on TV, so sorry if I just said something totally wrong and silly) , I am thinking of it just as an addition. So maybe it’s still not that stupid 🙂