php如何继承一个应用？

class operationLimit 
// operating limit. When no user login or is not this user 
{ 
/* for limit the user operat at post. 
* @author:xiaoai 8.12 2011 
*/ 
static $limitObject; 
public function construct() {} 
// when call the function but does not exist 
public static function getObject() 
{ 
if( !(self::$limitObject instanceof self)) 
self::$limitObject = new self; 
return self::$limitObject ; 
} 
protected function setLimit() {} 
public function getReadA($postName) 
{ 
return '<a herf=\'http://foodstory.me/post/'.$postname. 
'.php\' class=\'readmoreLink\'>readmore</a>'; 
} 
} 
class operationUnlimit 
extends
 operationLimit 
// when is this user 
{ 
public static function getObject() 
{ 
if( !(self::$limitObject instanceof self)) 
self::$limitObject = new self; 
return self::$limitObject ; 
} 
public function getUpdateA($name) 
{ 
return '<a href=\'http://foodstory.me/post/'.$name. 
'.php?do=update\' id=\''.$name.'\' >update</a>'; 
} 
public function getDelectA($name) 
{ 
return '<a href=\'
javascript
:delectPOST('.$name 
.');\' id=\'delectPOST\' >delect</a>'; 
} 
} 
class LimitFactory 
{ 
public static function getLimitObject($userStatus) 
// $userStatus = $session->get_status(); 
{ 
switch ( $userStatus ) 
{ 
case 0: 
return operationLimit::getObject(); 
case 1: 
return operationUnlimit::getObject(); 
default: 
return limit::getObject(); 
} 
} 
}

$limitObj = LimitFactory::getLimitObject($session->get_status()); 
echo $limitObj->getReadA('hi'); 
echo $limitObj->getDelectA('hah');