用PHP编码的登录只是一次又一次地刷新

使用PHP和MySQL创建登录/注册系统。注册已经开始了，我认为它还可以。但是，我在登录页面上遇到了一些问题。

不管怎样，它只是刷新了页面，我不确定出了什么问题。这是我在表单上运行的loginaccount.php脚本：

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编辑：

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谢谢你们到目前为止的帮助！但我还是遇到了一个棘手的问题。现在，不管怎样，它仍然无法登录，尽管现在我实际上收到了我设置的错误消息。更新代码如下：

<?php
//Database Information
$dbhost = "";
$dbname = "";
$dbuser = "";
$dbpass = "";
//Connect to database
mysql_connect ( $dbhost, $dbuser, $dbpass)or die("Could not connect: ".mysql_error());
mysql_select_db($dbname) or die(mysql_error());
session_start();
$username = $_POST[‘username’];
$password = md5($_POST[‘password’]);
$query = "select * from registerusers where username='$username' and password='$password'";
$result = mysql_query($query);
if (mysql_num_rows($result) == 0) {
$error = "Incorrect login, please try again.";
include "login.php";
echo "<span class=error_message>".$error."</span>"; 
} else {
$_SESSION['username'] = "$username";
include "login.php";
echo "<span class=success_message>Welcome! You are now logged in.</span>";
}
?>