thinkphp会员登录有关问题

<?php<br />
session_start();<br />
header('Content-Type:text/html; charset=utf-8');<br />
class AdminAction extends Action{<br />
      public function index(){<br />
	  if(isset($_POST['username'])){<br />
	        if(isset($_POST['username'])&&isset($_POST['password'])){<br />
			    $db=M();<br />
				$select=$db->query("select * from developers where username=".$POST['username']." and password=".$_POST['password']."");<br />
				if($select){<br />
				    $_SESSION['admin']=$_POST['username'];<br />
					$this->redirect('Index/index','',2,'用户'.$_POST['username'].'登录成功！');<br />
                 }else{<br />
					$this->redirect('Index/index','',2,'用户名或者密码不正确');<br />
					}<br />
				}else{<br />
				    $this->redirect('Index/index','',2,'用户名或密码不能为空！');<br />
					}<br />
					}<br />
					$this->display();<br />
					}<br />
	<br />
    <br />
<br />
	}<br />
	?>