you can try array_reduce
:
$someVariable="someValue";
$result = array_reduce(array_merge($array1, $array2), function ($carry, $item) use ($someVariable) {
if (isset($carry[$item['category_id']])) {
$carry[$item['category_id']] = array_merge($carry[$item['category_id']], $item);
} else {
$carry[$item['category_id']] = $item;
}
return $carry;
}, array());
var_dump($result);