I am tasked with learning PHP, but there are many things I don't understand. For example, the concept of "variable functions" is not one I've seen anywhere else. There are many other examples, but for brevity, I found PHPWTF, which has many examples of PHP's idiosyncrasies.

Most other languages I've used have either a formal specification (e.g., Haskell 2010) or at least a research paper on their formal semantics (e.g., this for Javascript). However, I can't find anything comparable for PHP.

There is an official "language reference". However, it is very informal, reads like a wiki, and is missing entire sections (e.g., the section on syntax doesn't define the syntax at all). Confirming what I suspected, this guy tells me that there is no official specification, nor even a defined syntax.

Wikipedia has an article on "PHP syntax and semantics", but it only touches on the syntax, and barely mentions semantics.

One paper I've found on PHP is this paper on its assignment semantics. This is a very small fragment of the language and probably not much use to me without some context. There is also this paper on 'SaferPHP', which presumably has to work with some definition of PHP, though I couldn't see any.

Interpreters/compilers provide a semantics, so I thought to look at these. However, the Zend source is intimidating (though it does provide useful test cases), and HipHop runs to 2.7 million LoC. (I find it amazing that people have poured enormous effort into writing compilers for a language without ever writing something like a specification.)

I thought of looking at type systems for PHP for guidance, much like TypeScript provides some guidance for JavaScript. I found these tantalising slides on Hack, an optional type system for PHP. However, it's just slides, and the project seems to be an internal one at Facebook at this time.

Does anyone know of anything better than these poor man's semantics? Or does everyone just "learn by example"?