密码是什么啊该如何解决

密码是什么啊
/*****************************
* login.php 登录验证处理文件
*****************************/
session_start();
// 注销登录
if($_GET['action'] == 'logout'){
session_unregister("username");
exit('');
}

// 如果检测到已登录，直接跳转至管理页面
if(isset($_SESSION['username'])){
header("Location: http://".$_SERVER['HTTP_HOST'].rtrim(dirname($_SERVER['PHP_SELF']), '/\\')."/admin.php");
exit;
}
if($_POST){
require("./conn.php");
$username = $_POST['username'];
$password = MD5(trim($_POST['password']));
$check_result = mysql_query("SELECT uid FROM user WHERE username = '$username' AND password = '$password'");
if(mysql_fetch_array($check_result)){
session_register("username");
header("Location: http://".$_SERVER['HTTP_HOST'].rtrim(dirname($_SERVER['PHP_SELF']), '/\\')."/admin.php");
exit;
} else {
echo '';
}
}

?>















------解决方案--------------------
$password = MD5(trim($_POST['password']));