get_students($id, $name = null, $email = null)

function get_students($id, $name = null, $email) {
  if (!empty($id)) {
    // Get students by their ids
  } else if (!empty($name) && !empty($email)) {
    // Get students by their names and emails
  }
}

get_students(1); //Will retrieve studen with id 1
get_students(null, "Name", "email@email.com"); //Will retrieve students with name "Name" and email "email@email.com"