$clsDatabase = new clsDatabase();
$user = new clsUser($clsDatabase);

class clsDatabase{
    private $host;
    private $user;
    private $pass;
    private $dbname;
    private $dbh;
    private $error;

    /**
     * Database constructor.
     */
    public function __construct(){
        $this->host = 'localhost';
        $this->user = '1';
        $this->pass = '1';
        $this->db = '1';
    }

    /**
     * Database instance.
     */
    public function getDb(){
        // Set DSN
        $dsn = 'mysql:host=' . $this->host . ';dbname=' . $this->dbname;
        // Set options
        $options = array(
            PDO::ATTR_PERSISTENT => true,
            PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
            PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES UTF8'
        );
        // Create a new PDO instanace
        try {
            $this->dbh = new PDO($dsn, $this->user, $this->pass, $options);
        } // Catch any errors
        catch (PDOException $e) {
            $this->error = $e->getMessage();
        }
    }
}

class clsUser{
    private $dbh;
    public function __construct(clsDatabase $clsDB){
         $this->dbh = $clsDB->getDb();
    }
}