Adding values of two multidimensional associative arrays in PHP

I am trying to sum the values of two associative arrays. This is the first array:

Array
(
[Jan 01, 2013] => Array
    (
        [COM] => 100
        [RES] => 200
    )

[Oct 28, 2014] => Array
    (
        [COM] => 300
        [RES] => 400
    )
)

and this is the second array:

Array
(
[Jan 01, 2013] => Array
    (
        [COM] => 10
        [RES] => 20
    )

[Oct 28, 2014] => Array
    (
        [COM] => 30
        [RES] => 40
    )
)

I need to sum the values from these two arrays so that it produces this result:

    Array
(
[Jan 01, 2013] => Array
    (
        [COM] => 110
        [RES] => 220
    )

[Oct 28, 2014] => Array
    (
        [COM] => 330
        [RES] => 440
    )
)

I found this tutorial to sum values of associative arrays, but it does not seem to work with my multi-dimensional arrays. Any suggestions how to accomplish this? Thank you.

---

Use foreach() for first array & check the key of first array whether it exist or not in second array. If exist the calculate sum. Example:

$arr1 = Array
(
    "Jan 01, 2013" => Array
    (
        "COM" => 100,
        "RES" => 200,
    ),

    "Oct 28, 2014" => Array
    (
        "COM" => 300,
        "RES" => 400,
    )
);
$arr2 = Array
(
    "Jan 01, 2013" => Array
    (
        "COM" => 10,
        "RES" => 20,
    ),

    "Oct 28, 2014" => Array
    (
        "COM" => 30,
        "RES" => 40,
   )
);

$arr3 = array();
foreach($arr1 as $key => $val):
    if(array_key_exists($key, $arr2)):
        $arr3[$key]["COM"] = $val["COM"] + $arr2[$key]["COM"];
        $arr3[$key]["RES"] = $val["RES"] + $arr2[$key]["RES"];
    endif;
endforeach;

print '<pre>';
print_r($arr3);
print '</pre>';

---

Output:

Array
(
    [Jan 01, 2013] => Array
        (
            [COM] => 110
            [RES] => 220
        )

    [Oct 28, 2014] => Array
        (
            [COM] => 330
            [RES] => 440
        )

)