PHP design mode Decorator (decoration mode)

Copy the code The code is as follows:
/**
* Decoration mode
*
* Dynamically add some additional responsibilities to an object, which is more flexible than generating subclasses in terms of extended functionality
*/
header("Content-type:text/html;charset=utf-8");
abstract class MessageBoardHandler
{
public function __construct (){}
abstract public function filter($msg);
}

class MessageBoard extends MessageBoardHandler
{
public function filter($msg)
{
return "Processing the content on the message board|".$msg;
}
}

$obj = new MessageBoard();
echo $obj->filter("Be sure to learn the decoration mode
");

// --- The following is how to use the decoration mode- ---
class MessageBoardDecorator extends MessageBoardHandler
{
private $_handler = null;

public function __construct($handler)
{
parent::__construct();
$this->_handler = $handler;
}

public function filter($msg)
{
return $this->_handler->filter($msg);
}
}

// Filter html
class HtmlFilter extends MessageBoardDecorator
{
public function __construct($ handler)
{
parent::__construct($handler);
}

public function filter($msg)
{
return "Filter out HTML tags|".parent::filter($msg);; // Filter The process of removing HTML tags is just to add text without processing
}
}

// Filter sensitive words
class SensitiveFilter extends MessageBoardDecorator
{
public function __construct($handler)
{
parent::__construct($handler ; Processing
}
}

$obj = new HtmlFilter(new SensitiveFilter(new MessageBoard()));
echo $obj->filter("Be sure to learn the decoration mode!
");