SKoschnicke

<?php

$a = array(1 => array(1),2 => array(2),3 => array(3), 12 => array(5));

$b = array(1 => 5,2 => 6,3 => 7, 5 => 12);

$result = $a + array_fill_keys(array_keys($b), []);
foreach ($b as $key => $value) {
  array_push($result[$key], $value);
}