Group an associative array by prefix to create a new array and by suffix to create a second new array

I have an array:

array(
 "A1" => 4,
 "A2" =>5,
 "B1" => 2,
 "B2" =>7,
 "C1" => 4,
 "C2" =>5,
 "D1" => 4,
 "D2" =>5,
  .....
)

Now I just want to new array:

$arr1 = array("A"=>array("A1"=>4, "A2"=>5), "B" => array("B1" => 2, "B2" => "7"), ...)

$arr2 = array(
 0=>array("A1"=>"4","B1"=>2,"C1"=>4,"D1"=>4),
 1=>array("A2"=>"4","B2"=>2,"C2"=>4,"D2"=>4)
)

How can I group the data in these two ways?

$arr = array(
 "A1" => 4,
 "A2" =>5,
 "B1" => 2,
 "B2" =>7,
 "C1" => 4,
 "C2" =>5,
 "D1" => 4,
 "D2" =>5,
);

ksort($arr, SORT_NATURAL);
$arr1 = $arr2 = array();
foreach ($arr as $key => $value) {
    $letter = substr($key, 0, 1);
    $index = (int)substr($key, 1) - 1;

    !isset($arr1[$letter]) && $arr1[$letter] = array();
    $arr1[$letter][$key] = $value;

    !isset($arr2[$index]) && $arr2[$index] = array();
    $arr2[$index][$key] = $value;
}

ksort($arr1, SORT_NATURAL);
ksort($arr2, SORT_NATURAL);

var_dump($arr1, $arr2);

Outputs:

array(4) {
  ["A"]=>
  array(2) {
    ["A1"]=>
    int(4)
    ["A2"]=>
    int(5)
  }
  ["B"]=>
  array(2) {
    ["B1"]=>
    int(2)
    ["B2"]=>
    int(7)
  }
  ["C"]=>
  array(2) {
    ["C1"]=>
    int(4)
    ["C2"]=>
    int(5)
  }
  ["D"]=>
  array(2) {
    ["D1"]=>
    int(4)
    ["D2"]=>
    int(5)
  }
}
array(2) {
  [0]=>
  array(4) {
    ["A1"]=>
    int(4)
    ["B1"]=>
    int(2)
    ["C1"]=>
    int(4)
    ["D1"]=>
    int(4)
  }
  [1]=>
  array(4) {
    ["A2"]=>
    int(5)
    ["B2"]=>
    int(7)
    ["C2"]=>
    int(5)
    ["D2"]=>
    int(5)
  }
}