在分配的函数返回数组上调用数组键？

Question

From PHP 5.4, I can call an element of a function returned array, like so: func()['el'].

I'm running PHP 5.6 and test the following (simplified) code:

Works

$decoded = json_decode($content, true)['foo'];

Doesn't Work

($decoded = json_decode($content, true))['foo']; // syntax error ['

I don't understand why I cannot assign json_decode($content, true) to $decoded and call the element foo immediately. This works fine in php 7

Why this is Interesting?

My PhpStorm 2017.2.1, with applied php 5.6 DOESN'T detect this code as available in PHP 7 only!

Answer 1

One of the many features that PHP 7 introduced was an Abstract Syntax Tree. This directly led to more complex expressions being possible, such as the one in your question.

Simply, the change is that a function return wrapped in brackets previously did not behave the same as one that is not:

<?php
function foo() { return ['a' => 'b']; }

echo foo()['a']; // Works in PHP 5.4+
echo (foo())['a']; // Works in PHP 7+

Functional array dereferencing, as you mention, was added in 5.4, but only as a special case in the parser. In PHP 7+, you can dereference any expression that yields an array.