class DB { // this class ONLY interacts with the DB
    private $link = null;
    public function __construct($host, $username, $password, $db) {
        $this->link = new PDO('mysql:host='.$host.';dbname='.$db, $username, $password);
    }

    public function doQuery($sql, $values) {
        // do the query using PDO etc
    }
}

class Animal {
    private $db = null;
    public function __construct(\DB $db) { // here we typehint for the db class we need
        $this->db = $db;                   // but this class has no idea how to construct it
    }

    public function poop() {
        // all animals do it, basically constantly.
    }

    public function save() {
        // use $this->db to write to database
    }
}

class Dog extends Animal {
    public $breed = 'Mutt';
    public function __construct(\DB $db, $breed = 'Mutt') {
        parent::_construct($db);
        $this->breed = $breed;
        $this->poop(); // dogs...
    }
}

// make the db connection
$db = new DB('localhost', 'user', 'password', 'animalDb');

// pass it to the class that will use it.
$myDog = new Dog($db, 'German Shorthair Pointer');