PHP equivalent of friend or internal

PHP doesn't support friend-like declarations. It can be simulated in PHP5 using the __get and __set methods and by inspecting a backtrace for the allowed friend classes. But this type of coding practice is considered to be clumsy −

class sample_friend {
   private $__friends = array('My_Friend', 'Other_Friend');
   public function __get($key)    {
      $trace = debug_backtrace();
      if(isset($trace[1]['class']) && in_array($trace[1]['class'], $this->__friends)) {
         return $this->$key;
      }
      // __get() code goes here
      trigger_error('Cannot access private property ' . __CLASS__ . '::$' . $key, E_USER_ERROR);
   }
   public function __set($key, $value) {
      $trace = debug_backtrace();
      if(isset($trace[1]['class']) && in_array($trace[1]['class'], $this->__friends)) {
         return $this->$key = $value;
      }
      // normal __set() code goes here
      trigger_error('Cannot access private property ' . __CLASS__ . '::$' . $key, E_USER_ERROR);
   }
}