Description:
------------
Accessing an array will a null index is equal to accessing it with an empty string index, which is non-trivial and cause of many errors in applications.

When a developer writes 

$a[$index]=5;

They typically expect $index to be an index (regardless of type) and not null. But if it is null, the command still works and is equal to:

$a[""]=5; //same as $a[null]=5;

There is no trivial reason why null should cast to empty string, rather than say 0 or any other equivalent form. And this behavior typically should cause warnings or notices (although it breaks BC).


Test script:
---------------
$a=[];
$a[]=4;
$a[null]=5;
$a[]=6;
var_dump($a);

Expected result:
----------------
E_NOTICE, E_WARNING or $a[0]=5, but not $a[""]=5


Actual result:
--------------
array(4) {
  [0]=>
  int(4)
  [""]=>
  int(5)
  [1]=>
  int(6)
  [2]=>
  int(7)
}