gistfile1.php

$array = array(
  array('id' => 'a', 'value' => 11),
  array('id' => 'b', 'value' => 12),
  array('id' => 'c', 'value' => 13),
  array('id' => 'a', 'value' => 21),
  array('id' => 'b', 'value' => 22),
  array('id' => 'b', 'value' => 32),
);
var_dump(
  array_reduce($array, function ($result, $item) {
    $result[$item['id']][] = $item;
    return $result;
  }, array())
);

/*
array(3) {
  ["a"]=> array(2) {
    [0]=> array(2) { ["id"]=> string(1) "a" ["value"]=> int(11) }
    [1]=> array(2) { ["id"]=> string(1) "a" ["value"]=> int(21) }
  }
  ["b"]=> array(3) {
    [0]=> array(2) { ["id"]=> string(1) "b" ["value"]=> int(12) }
    [1]=> array(2) { ["id"]=> string(1) "b" ["value"]=> int(22) }
    [2]=> array(2) { ["id"]=> string(1) "b" ["value"]=> int(32) }
  }
  ["c"]=> array(1) {
    [0]=> array(2) { ["id"]=> string(1) "c" ["value"]=> int(13) }
  }
}
*/