Description:
------------
The following two pieces of code have the same output.

i want to know is that the correct feature of php or something?

Test script:
---------------
$b=1;
$a=array(1=>"maple");
$a[$b+1]=$b=2;
var_dump($a);

$b=1;
$a=array(1=>"maple");
$a[$b]=$b=2;
var_dump($a);

Expected result:
----------------
array(2) {
  [1]=>
  string(5) "maple"
  [3]=>
  int(2)
}
array(2) {
  [1]=>
  string(5) "maple"
  [2]=>
  int(2)
}


Actual result:
--------------
array(2) {
  [1]=>
  string(5) "maple"
  [2]=>
  int(2)
}
array(2) {
  [1]=>
  string(5) "maple"
  [2]=>
  int(2)
}