function missing_number($num_list)
{
 
// construct a new array
$new_arr = range($num_list[0],max($num_list));                                                    

// use array_diff to find the missing elements 
return array_diff($new_arr, $num_list);

}

print_r(missing_number(array(1,2,3,6,7,8)));
print_r(missing_number(array(10,11,12,14,15,16,17)));
?>