如何在PHP中将数组转换为二维数组

I'd like to convert this array:

Array
(
    [5] => 3.5
    [6] => 4.5
    [7] => 5.5

)
Array
(
    [8] => 5
    [9] => 6
    [10] => 7

)

Into a two-dimensional array that would look like this:

$array = array(
"a" => array("3.5","4.5", etc.),
"b" => array("5","6", etc.),
);

I think I'm getting close with the following code, but I'm still missing a name such as "a" and "b" for each array group:

$array = array($result);
echo '<pre>';
print_r(array_chunk($array,2, true));
echo '</pre>';

Here is the code from where I'm getting the two arrays :

$period = array(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15);
$sma = array(6,9);

foreach ($sma as $range)    {

    $sum = array_sum(array_slice($period, 0, $range));
    $result = array($range - 1 => $sum / $range);

    for ($i = $range, $n = count($period); $i != $n; ++$i) {

        $result[$i] = $result[$i - 1] + ($period[$i] - $period[$i - $range]) / $range;

    }

    $array = array($result);
    echo '<pre>';
    print_r(array_chunk($array,2, true));
    echo '</pre>';
} 

    $period = array(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15);

    $sma = array(6,9);

    $array = array();

    foreach ($sma as $range)    {

    $sum = array_sum(array_slice($period, 0, $range));

    $result = array($range - 1 => $sum / $range);

    for ($i = $range, $n = count($period); $i != $n; ++$i) {

    $result[$i] = $result[$i - 1] + ($period[$i] - $period[$i - $range]) / $range;

    }

//add each array to final array
    $array[] = $result;

    } 
    echo '<pre>';
    print_r($array);
    echo '</pre>';