将关联数组的数组与缺少键的空字符串值合并

I have two arrays of associative arrays with different keys. I need to merge them into a single array of associative arrays, with nulls or empty strings for keys that do not exist at higher indices. For example:

$first = array(array('x'=>'1','y'=>'2'));
$second = array(array('z'=>'3'),array('z'=>'4'));

The result should look like this:

$result = array(
    array(
        'x'=>'1',
        'y'=>'2',
        'z'=>'3'
        ),
    array(
        'x'=>'',
        'y'=>'',
        'z'=>'4'
        )
    );

The function that merges these arrays needs to be able to handle two or more arrays. Here's what I came up with:

// allArrays can be many arrays of all sizes and will be different each time this process runs
$allArrays = array($first, $second);
$longestArray = max($allArrays);
$data = array();
for($i = 0; $i < count($longestArray); ++$i) {
    $dataRow = array();
    foreach ($allArrays as $currentArray) {
        if (isset($currentArray[$i])) {
            foreach ($currentArray[$i] as $key => $value) {
                $dataRow[$key] = $value;
            }
        } else {
            foreach ($currentArray[0] as $key => $value) {
                $dataRow[$key] = '';
            } 
        }                
    }
    $data[] = $dataRow;
}

It works, but I think the nested for loops cause poor performance on large arrays, and it's pretty illegible. Is there a better way to tackle this problem?