php - create,dynamically, an array initialized with N null elements

Actually a call to array_fill should be sufficient:

//...
public function create_array($num_elements){
    return array_fill(0, $num_elements, null);
}
//..

var_dump(create_array(3));
/*
array(3) {
  [0]=> NULL
  [1]=> NULL
  [2]=> NULL
}
*/

for ($i = 0; $i < $num_elements; $i++) {
    $array[$i] = null;
}