如何在php中的数组中添加缺失的索引？

function fillMissingIndexes($array) {
    $keys = array_keys($array);
    sort($keys);
    $newArray = array();
    $lastKey = 0;
    foreach ($keys as $key) {
        for ($i = $lastKey + 1; $i < $key; $i++) {
            $newArray[$i] = null;
        }
        $newArray[$key] = $array[$key];
        $lastKey = $key;
    }
    return $newArray;
}

$array = array(
    0 => 'a',
    1 => 'b',
    3 => 'c',
    5 => 'd'
);

$arrayWithFilledKeys = fillMissingIndexes($array);
print_r($arrayWithFilledKeys);

Array
(
    [0] => a
    [1] => b
    [2] => 
    [3] => c
    [4] => 
    [5] => d
)

$array1 = array(
    0 => 'a',
    1 => 'b'
);

$array2 = array(
    2 => 'c',
    3 => 'd'
);

$filledArray = array_merge($array1, $array2);
sort($filledArray);
print_r($filledArray);

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
)