class self_test {
  static $instance;
  public function __construct(){
    self::$instance = 'instance';//静态属性只能通过self来访问
  }
  public function tank(){
    return self::$instance;//访问静态属性
  }
}
$str = new self_test();
echo $str->tank();

class self_test {
  static $instance;
  public function __construct(){
    self::$instance = 'dell';
  }
  static public function pentium(){
    return self::$instance;//静态方法也可以继续访问静态变量，访问时需要加$
  }
  public function tank(){
    return self::pentium();//访问静态方法pentium()
  }
}
$str = new self_test();
echo $str->tank();

class self_test {
  const NAME = 'tancy';
  public function tank(){
    return self::NAME;
  }
}
$str = new self_test();
echo $str->tank();

class self_test {
  public $public;
  private $private;
  protected $protected;
  public function __construct(){
    $this->public = 'public';
    $this->private = 'private';
    $this->protected = 'protected';
  }
  public function tank(){
    return $this->public;
  }
  public function dell(){
    return $this->private;
  }
  public function datesrt(){
    return $this->protected;
  }
}
$str = new self_test();
echo $str->tank();
echo "</br>";
echo $str->dell();
echo "</br>";
echo $str->datesrt();