Application1.php

$user = new User($stoage);

Application2.php

<?php

$storage = new DatabaseStorage($host, $username, $password);
$user = new User($stoage);

DatabaseStorage.php

<?php

Class DatabaseStorage extends StorageInterface {

  public function insert($bin, array $data) {
    // Query to insert into bin/table.
  }
}

StorageInterface.php

<?php


Interface StorageInterface {
  public function insert($data);
  public function delete();
  public function update();
}

User.php

<?php

Class User {
  public function add($uid, $name) {
    $storage = new DatabaseStorage();
    $storage->insert('user', array($uid, $name));
  }
}

User2.php

<?php

Class User {
  protected $stoage;

  public function __construct(StorageInterface $storage) {
   $this->storage = $storage;
  }
  public function add($uid, $name) {
    $this->storag->insert('user', array($uid, $name));
  }
}