<?php
class Post {
  private $title;
  private $content;
  private $author;
  private $comments;

  private $_getters = array('title', 'content', 'author', 'comments');
  private $_setters = array('title', 'content', 'author');
  
  public function __get($property) {
    if (in_array($property, $this->_setters)) {
      return $this->$property;
    }
    else if (method_exists($this, '_get_' . $property))
      return call_user_func(array($this, '_get_' . $property));
    else if (in_array($property, $this->_getters) OR method_exists($this, '_set_' . $property))
      throw new Exception('Property "' . $property . '" is write-only.');
    else
      throw new Exception('Property "' . $property . '" is not accessible.');
  }

  public function __set($property, $value) {
    if (in_array($property, $this->_getters)) {
      $this->$property = $value;
    }
    else if (method_exists($this, '_set_' . $property))
      call_user_func(array($this, '_set_' . $property), $value);
    else if (in_array($property, $this->_setters) OR method_exists($this, '_get_' . $property))
      throw new Exception('Property "' . $property . '" is read-only.');
    else
      throw new Exception('Property "' . $property . '" is not accessible.');
  }
}
?>

This way the variables in the $_getters array can be read from the outside and the variables in the $_setters array can be modified from the outside, like this:

<?php
$post = new Post();
$post->title = 'Hello, World';
echo $post->title;

// The following will throw an exception since $comments is read-only:
$post->comments = 23;
?>

And in case you need a less generic getter or setter at some point, you can remove the variable from the $_getters or $_setters array and implement a method like:

<?php
private function _set_title($value) {
  $this->title = str_replace('World', 'Universe', $value);
}
?>

And from the outside the property could still be used with:

<?php
$post->title = 'Hello, World!';
?>
 

 上面是手冊裡的例子，但是我覺得應該是先判斷類裡面是否有處理屬性的方法，有的話就呼叫該方法，沒有就直接設定該屬性。