PHP数组结构$ arr [（$ arr [$ arr]）]解释

Question

I've been given some code and one line is:

 $key = $arr[($arr[$arr])];

I've never seen () in [] before, and Googling around I can't seem to find an explanation.

I don't know the value of $arr, but I wish to set it.

Doing:

$arr = 'blah';
$key = $arr[($arr[$arr])]; 

gives me Warning: Illegal string offset 'blah'

So I cannot print_r on it to find out what it looks like.

Answer 1

In general, there's nothing magical or strange here; the expression ($arr[$arr]) is equivalent to the expression $arr[$arr], just as (2) is equivalent to 2. At worst, the parens are redundant.

That being said, keys cannot have array-type, so trying to look up an element in an array using itself as a key is .... meaningless....