Use array_merge()
. It is an extra step but since you can't do this during the array declaration it is the next best thing.
$new_array = array_merge($arr, array('four' => 4, 'five' => 5));
print_r($new_array);
Array ( [one] => 1 [two] => 2 [three] => 3 [four] => 4 [five] => 5 )