Wie gruppiere und summiere ich Daten in einem 2D-Array nach Spalten in PHP?

[
    ['quantity' => 5,  'dd' => '01-Nov-2012'],
    ['quantity' => 10, 'dd' => '01-Nov-2012'],
    ['quantity' => 3,  'dd' => '02-Nov-2012'],
    ['quantity' => 4,  'dd' => '03-Nov-2012'],
    ['quantity' => 15, 'dd' => '03-Nov-2012'],
]

[
    ['quantity' => 15, 'dd' => '01-Nov-2012'],
    ['quantity' => 3,  'dd' => '02-Nov-2012'],
    ['quantity' => 19, 'dd' => '03-Nov-2012'],
]

// Input array
$in = array(array());

// Create output array
$out = array();

// Iterate through input rows
foreach ($in as $row) {
    // Check if 'dd' key exists in output array
    if (!isset($out[$row['dd']])) {
        // Initialize entry for 'dd' with empty quantity
        $out[$row['dd']] = array(
            'dd' => $row['dd'],
            'quantity' => 0,
        );
    }

    // Accumulate quantity for 'dd' key
    $out[$row['dd']]['quantity'] += $row['quantity'];
}

// Adjust output array to numerical indexing
$out = array_values($out);

// Display output array
var_dump($out);