PHP: group (multidimensional, associative) array and sum values ​​by specific key

I've searched a lot of examples with group/sum array values - unfortunately I can't resolve my problem. I got array $data which looks like this:

Array
(
    [0] => Array
        (
            [0] => 1
            [1] => 2014
            [context] => 'aaa'
        )

    [1] => Array
        (
            [0] => 12
            [1] => 2014
            [context] => 'aaa'
        )       

    [2] => Array
        (
            [0] => 5
            [1] => 2014
            [context] => 'zzz'
        )
)

I would like to group and sum its values (but not all) by 'context'.

So desired output is:

Array
(
    [0] => Array
        (
            [0] => 13
            [1] => 2014
            [context] => 'aaa'
        )

    [1] => Array
        (
            [0] => 5
            [1] => 2014
            [context] => 'zzz'
        )       

)

I'm far from this expected output. I've tried something like:

$result = array();
foreach ($data as $subArray)
{
  foreach ($subArray as $row)
  {
    $result[$row['context']] = $row['context'];
    $result[$row['1']] = $row['1'];
    $result[$row['0']] += $row['0'];
  }
}

But of course it doesn't work and I'm out of ideas. Can you please give me a hint? What else can I try?

---

The problem is you were overwriting elements in your loop and you were counting on an extra nesting level:

$data = array(
    0 => array(
            0 => 1,
            1 => 2014,
            'context' => 'aaa'
        ),

    1 => array(
            0 => 12,
            1 => 2014,
            'context' => 'aaa'
        ),       

    2 => array(
            0 => 5,
            1 => 2014,
            'context' => 'zzz'
        )
);

$result = array();

// the elements to sum - since everything is mixed together.
// the values in this array should be the index you want to sum
// ex. $sum = array(2,3) would sum $data[2] and $data[3]
$sum = array(0);

foreach ($data as $subArray)
{
  $context = $subArray['context'];

  if (!isset($result[$context])) {
    // initialize the result for this context because it doesnt exist yet
    $result[$context] = array();
  }

  // you had an extra nesting level here
  // $row was equiv to 'aaa' or '2014' whereas you thought it was
  // array('context' => 'aaa', 0 => 5, 1 => '2014')
  foreach ($subArray as $idx => $val)
  {
    // you were also constantly overrwriting $result'aaa'] (or whatever context) here
    if (in_array($idx, $sum)) {
       $result[$context][$idx] = isset($result[$context][$idx])
         ? $result[$context][$idx] + $val
         : $val;
    } else {
      // this will keep overwriting anything that isnt in the $sum array
      // but thats ok because those values should be the same with, or the last
      // one should win. If you need different logic than that then adjsut as necessary
      $result[$context][$idx] = $val;
    }
  }
}

printf('<pre>%s</pre>', print_r($result, true));