Data Access Objects (DAOs) are a common pattern for persisting Domain objects into the database. It's easy to confuse the DAO pattern with a Repository. The significant difference is that Repositories represent collections, while DAOs are closer to the database and are often far more table-centric. Typically, a DAO would contain CRUD methods for a particular Domain object. Let's see how a common interface for a DAO might look:

interface UserDAO 
{ 
    /** 
     * @param string $username 
     * @return User 
     */ 
    public function get($username);

    public function create(User $user);

    public function update(User $user);

    /**
     * @param string $username
     */
    public function delete($username);
}

A DAO interface could have multiple implementations, which could range from using ORM constructions to using plain SQL queries. The main problem with DAOs is that their responsibilities are not clearly defined. DAOs are usually perceived as gateways...