Chike Mgbemena has a new post to his site looking at PHP 7 and the abstract syntax tree and uniform variable syntax changes that came along with it.

On my previous post (PHP 7 In-depth Look), I discussed in-depth about the features of PHP 7 (you can read it here if you have not). In this post, I am going to be talking about The Abstract Syntax Tree(AST)/Uniform Variable Syntax in PHP 7+.

PHP 7 introduced a new layer which is called the Abstract Syntax Tree(AST) which helps in decoupling the process of parsing from the pseudo-compile process. Note that this new layer does not have much impact on performance but it make the syntax uniform. Uniform variable syntax/abstract syntax tree aims to establish internally consistent variable syntax, references are accessed from left to right instead of right to left.

He goes on to talk about dereferencing, how it changed from the PHP 5 handling and what IIFEs have to do with it. Some sample code is included showing some of his points and how PHP 7 interprets things slightly different than PHP 7.

Davey Shafik has posted a quick article to his site talking about class constants and something new he learned about them (and how it relates to the uniform variable syntax handling in PHP7).

Yesterday on Twitter there was a conversation started by Marco Pivetta regarding a particularly horrible bit of code he had spotted [that] creates a string using sprintf() by prefixing ::PARAMNAME with the result of calling get_class() on the $api variable, and then passes that string into constant() which will give you the value of a constant using it’s string name.

The conversation continued with comments from Elizabeth Smith about why this workaround was needed in the past. Davey also suggests that it won't work as expected if the input is an object and not a string but a test from Trevor Suarez proved that incorrect as well (it does work). He ends the post talking about PHP7 and showing how, thanks to the uniform variable syntax changes, this same kind of handling can be done in many other ways too.

There's been an RFC that's recently made it through the voting process and was approved for inclusion in PHP6, the uniform variable syntax handling. When these changes are put into effect, some of the odd syntax you had to use for things like variable variables will be cleared up and standardized. However, Derick Rethans stood out as the only "no" vote, here's why...

As you might have heard, PHP developers voted on an RFC called "Uniform Variable Syntax". This RFC "proposes the introduction of an internally consistent and complete variable syntax". In general, this RFC argues for making PHP's parser more complete for all sorts of variable dereferences. [...] Thirty people voted for, and one against: Me. Does that mean that I am against a unified variable syntax? No, I am not. I am actually quite a fan of having a consistent language, but we need to be careful when this hits existing users.

He points out that there's known backwards compatibility breaks in the changes and this breaks the semantics of the language. While the BC breaks are understood, Derick suggests that this is one of the worst changes a language can make: "...and this is exactly why people whine that PHP breaks BC and does not care about its users".