Group rows of a 2d array by a column and sum individual columns within each group

I have an array that I get back from an SQL query and it looks like this when I do print_r:

Array(
    [0] => Array(
        [id] => 1,
        [protein] => 20,
        [carbs] => 64,
        [date] => 2014-02-24
    ),
    [1] => Array(
        [protein] => 1,
        [carbs] => 12,
        [score2] => 8,
        [date] => 2014-02-24
    ),
    [2] => Array(
        [id] => 1,
        [protein] => 47,
        [carbs] => 84,
        [date] => 2014-02-25
    )
)

Now what I would like to do to this array is add up the carbs and protein values for each particular date. So take day 2014-02-24 I would like it for the carb and protein values to equal to 21 and 76. So far I can access each individual value in a foreach loop but I am not sure how I can save these to a specific date. Here is the loop I am using:

foreach ($res as $row) {
   echo $row['protein'] . "<br />" . $row['carbs'] . "<br />" . $row['date'] . "<br />";
}

So obviously this doesn't give me the results I want but I am not sure how to group the information by a certain date, can someone explain how I can do this?

You need to loop over the array and add the totals for each carbs and protein to a variable or array representing each date to get the total for that date:

$grouped = array();
foreach($res as $row) {
    if(!array_key_exists($row['date'], $grouped)) // create array key if it doesnt exist
        $grouped[$row['date']] = array('protein' => 0, 'carbs' => 0);

    $grouped[$row['date']]['protein'] += $row['protein']; // sum for each date
    $grouped[$row['date']]['carbs'] += $row['carbs'];
}

print_r($grouped);

Example output:

Array(
    [2014-02-24] => Array(
        [protein] => 21
        [carbs] => 76
    )
    [2014-02-25] => Array(
        [protein] => 47
        [carbs] => 84
    )
)