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']
];