php数组实现根据某个键值将相同键值合并生成新二维数组的方法

$infos = array(
  array(
    'a' => 36,
    'b' => 'xa',
    'c' => '2015-08-28 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  ),
  array(
    'a' => 3,
    'b' => 'vd',
    'c' => '2015-08-20 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  ),
  array(
    'a' => 6,
    'b' => 'wwe',
    'c' => '2015-08-28 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  ),
  array(
    'a' => 36,
    'b' => 'se',
    'c' => '2015-08-28 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  ),
  array(
    'a' => 6,
    'b' => 'aw',
    'c' => '2015-08-28 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  ),
  array(
    'a' => 36,
    'b' => 'bv',
    'c' => '2015-08-28 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  ),
  array(
    'a' => 12,
    'b' => 'xx',
    'c' => '2015-08-27 00:00:00',
    'd' => '2015/08/438488a00b3219929282e3652061c2e3.png'
  )
);
处理要求：将数组中a相同的二维数组合并到一个数组中，生成一个新的数组

<?php
 $result= array();
 foreach ($infos as $key => $info) {
  $result[$info['a']][] = $info;
 }
 print_r($result);

Array
(
 [36] => Array
  (
   [0] => Array
    (
     [a] => 36
     [b] => xa
     [c] => 2015-08-28 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
   [1] => Array
    (
     [a] => 36
     [b] => se
     [c] => 2015-08-28 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
   [2] => Array
    (
     [a] => 36
     [b] => bv
     [c] => 2015-08-28 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
  )
 [3] => Array
  (
   [0] => Array
    (
     [a] => 3
     [b] => vd
     [c] => 2015-08-20 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
  )
 [6] => Array
  (
   [0] => Array
    (
     [a] => 6
     [b] => wwe
     [c] => 2015-08-28 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
   [1] => Array
    (
     [a] => 6
     [b] => aw
     [c] => 2015-08-28 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
  )
 [12] => Array
  (
   [0] => Array
    (
     [a] => 12
     [b] => xx
     [c] => 2015-08-27 00:00:00
     [d] => 2015/08/438488a00b3219929282e3652061c2e3.png
    )
  )
)