Merge and transpose multiple flat associative arrays into a 2d array with null values where column values are missing

I need to get two arrays to merge into one while keeping all the keys in place and listing the values in an array like in this example:

$array1 = [
    'car' => '3',
    'bus' => '2'
];
$array2 = [
    'dog' => '1',
    'car' => '2',
    'bird' => '9'
];  

Desired result:

$merged = [
    'car' =>  ['3',  '2'], 
    'bus' =>  ['2',  null],
    'dog' =>  [null, '1'],
    'bird' => [null, '9']
];

function merge_common_keys(){
    $arr = func_get_args();
    $num = func_num_args();

    $keys = array();
    $i = 0;
    for ($i=0; $i<$num; ++$i){
        $keys = array_merge($keys, array_keys($arr[$i]));
    }
    $keys = array_unique($keys);

    $merged = array();

    foreach ($keys as $key){
        $merged[$key] = array();
        for($i=0; $i<$num; ++$i){
            $merged[$key][] = isset($arr[$i][$key]) ? $arr[$i][$key] : null;
        }
    }
    return $merged;
}

Usage:

$merged = merge_common_keys($array1,$array2);

PS. It can work with more than two arrays, just pass as many as you want as next arguments.