函数参数NULL有关问题

<br /><br />	class DB_Connect{<br />		<br />		protected $db;<br />		<br />		protected function __construct($dbo=NULL)<br />		{<br />			if(is_object($dbo))<br />			{<br />				$this->db=$dbo;				<br />			}<br />			else<br />			{<br />				$dsn="mysql:host=".DB_HOST."; dbname=".DB_NAME;<br />				try <br />				{<br />					$this->db=new PDO($dsn,DB_USER,DB_PASS);<br />				}<br />				catch(Exception $e)<br />				{<br />					die($e->getMessage());<br />				}<br />			}<br />		}		<br />	<br />	}	<br />