UPDATE:

Thanks to tim's answer, I have an improved version.

These are the new methods:

public function get_user_by_id($db_con, $id){
  $query_get_user = $db_con->prepare('SELECT * FROM users WHERE user_id=:id');
  $query_get_user->bindValue('id', $id, PDO::PARAM_STR);
  $query_get_user->execute();
  return $query_get_user->fetch();
}
public function get_user_by_username($db_con, $username){
  $query_get_user = $db_con->prepare('SELECT * FROM users WHERE user_username=:username');
  $query_get_user->bindValue('username', $username, PDO::PARAM_STR);
  $query_get_user->execute();
  return $query_get_user->fetch();
}

And how I use them in the controller:

public function get_user($get_user_variable, $get_user_value){
  $get_user = false;
  if($get_user_variable == "id")
    $get_user = $this->user->get_user_by_id($this->db_con, $get_user_value);
  elseif($get_user_variable == "username")
    $get_user = $this->user->get_user_by_username($this->db_con, $get_user_value);
  return $get_user;
}