第一回接触这种 查了好久也不会 = - 求解

<br /><?php<br />session_start();<br /><br /> class chkuserlogin{<br /> <br />    private $username;<br />	private $password;<br />    <br />    <br />	public function __construct($username,$password){<br />	   $this->username=$username;<br />	   $this->password=$password;<br />	   <br />	<br />	  <br />	   require_once("connn.php");<br />	   $sql=mysql_query("select account from root where username='".$this->username."' and password='".$this->password."'",$conn);      <br /> $info=mysql_fetch_array($sql);<br />	   if($info==false){<br />	     echo "<script>alert('用户名或密码输入错误！');history.back();</script>";<br />	     exit;<br />	   }else{<br />	     if($_SESSION["unc"]!=""){<br />		   session_unregister("unc");<br />		 } <br />		 session_register("unc");<br />		 $_SESSION["unc"]=$this->username; <br />		 mysql_query("update root set logintimes=logintimes+1,lastlogintime='".date("Y-m-d H:i:s")."'",$conn);<br />	     echo "<script>alert('登录成功！');window.location.href='login.php';</script>"; <br />	    }<br />	  }<br /> }<br /><br />  $obj=new chkuserlogin($_POST["username"],md5($_POST["password"]));<br />  $obj->chk();<br />?><br />