Group values of a flat array by the corresponding values of another flat array

Question

I have two arrays in PHP:

Array1
(
    [0] => 1
    [1] => 2
    [2] => 2
)
Array2
(
    [0] => 18
    [1] => 19
    [2] => 20
)

Array1 contains the Ids of Delivery Addresses. Array2 contains the Ids of Contacts.

Array1 and Array2 are 'aligned' so that Contact 18 (Array2[0]) resides at Delivery Address Id #1 (Array1[0]) (and so on).

What I would like is use the unique values of Array1 as array keys for Array3, and the values of Array2 used as the array values Array3.

The end result being that Contacts are 'grouped' by their Delivery Address.

Like so:

Array 3
(
   [1] = array (
                 [0] => 18
               )
   [2] = array (
                 [0] => 19
                 [1] => 20
               )
)

Answer 1

$array3 = array();
foreach ( $array1 as $k => $v ) {
    if ( !isset($array3[$v]) )
        $array3[$v] = array();

    $array3[$v][] = $array2[$k];
}

var_dump($array3);