Actually, ColdFusion did do something very similar to this ... the pages consist of XML-like tags interspersed with the HTML (which, of course, is also made up of tags).

As far as I could ever figure out about how the thing actually worked, it seemed that it was rather like a over-glorified XSLT of sorts.   And, it actually does work...   And it’s not “just PHP,” because the runtime engine obviously looks at the document structurally.   (The drawback is that eventually you get damn-tired of typing <CFeverything> <CFin> <CFthe> <CFfriggin> <CFworld>, <CFover> <CFand> <CFover> <CFagain>.   But, hey, it’s an acquired taste, I guess, and open-source implementations do exist.)

So, your essential idea, while not entirely new (what really is??), was viable enough to have been developed into a still very widely used application-server product.