A tip for PHP static

<?
class A
{
private $_count = "";
static private $_instance = null;
private function __construct( )
{
}
public function showCount()
{
echo $this->_count;
}
static public function setCount( $num )
{
$obj = self::getInstance();
$obj->_setCount($num);
return $obj;
}
private function _setCount( $num )
{
$this->_count += $num;
}
static private function getInstance()
{
if ( is_null(self::$_instance) )
self::$_instance = new self();
return self::$_instance;
}
}
A::setCount(5);
A::setCount(10)->showCount();
?>