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