file.php

Class User {

  public function listAll() {
    // This method is used on many classes.eg: Member class
  }

}

class Member {
  private $user;

  private function user() {
    if(! $this->user) {
      $this->user = new User;
    }
    return $this->user;
  }

  public function list() {
    return $this->user()->listAll();
  }

}