Question: 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 = array('car' => '3', 'bus' => '2');
$array2 = array('dog' => '1', 'car' => '2', 'bird' => '9');  

$merged = array(
    'car' => array('3','2'), 
    'bus' => array('2',null),
    'dog' => array(null,'1'),
    'bird' => (null,'9')
);

```

Answer: 
```
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.
Something like this? <http://php.net/array_merge_recursive>