PHP formal semantics?

Question

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"?

Answer 1

It seems that you're not after an official standard (which might be useful, for example, to someone writing an independent conforming implementation), but for a presentation of the language that will allow you to make coherent sense of it. Unfortunately there cannot be such a thing, because PHP does not have a coherent formal model behind it. It has grown organically and is now saddled with inconsistencies, most notoriously in function and method naming but also in little details like what counts as true and false, and other similarly worrisome details.

The best one can do to approach PHP, in my opinion, is to get a good feel for the core features and libraries, for the "gotcha's" that you need to watch out for, and (in order to read existing code without distraction) for the anti-patterns that are all too common in real-world PHP scripts. My guess is that it's best to learn PHP under the tutelage of people who know how to work with it effectively, but I didn't have that luxury. (Regarding the documentation: It took me forever before I noticed that you can use square brackets to index into strings. The feature may be mentioned somewhere in the documentation, but not, back then at least, anyplace where it belongs.)

This article gives a nice tour of the kind of things that make a semantic model of the kind you want impossible. (You may want to skip the opening rant and go straight to the discussion of PHP features.) There are many, many other similar texts. Quote: "PHP was originally designed explicitly for non-programmers (and, reading between the lines, non-programs); it has not well escaped its roots."

Don't get me wrong: I work with PHP, and although it's not my favorite language, I wouldn't say I hate it. I would say that to work effectively with it, one must be aware of its nature and limitations. If you're coming to this from Haskell, you're in for quite a shock.