Understanding differences in parsing

In PHP 5, expressions on the right side of an assignment operation were parsed right-to-left. In PHP 7, parsing is consistently left-to-right.

How to do it...

1. A variable-variable is a way of indirectly referencing a value. In the following example, first $$foo is interpreted as ${$bar}. The final return value is thus the value of $bar instead of the direct value of $foo (which would be bar):

   $foo = 'bar';
   $bar = 'baz';
   echo $$foo; // returns  'baz'; 

2. In the next example we have a variable-variable $$foo, which references a multi-dimensional array with a bar key and a baz sub-key:

   $foo = 'bar';
   $bar = ['bar' => ['baz' => 'bat']];
   // returns 'bat'
   echo $$foo['bar']['baz'];

3. In PHP 5, parsing occurs right-to-left, which means the PHP engine would be looking for an $foo array, with a bar key and a baz. sub-key The return value of the element would then be interpreted...