在php电子商务Web应用程序中创建登录表单

Question

我正在使用PHP创建一个电子商务Web应用程序。创建一个登录表单我遇到了问题。我提供了正确的用户名和密码，但仍然提供消息“无效的用户名或密码”。所以请有人帮忙。我正在提供表格和脚本。 脚本在这里：

<?php 
session_start();
if(isset($_SESSION['admin'])){
    header("location: admin/index.php");
    exit();
}
$msg = "";
if(isset($_POST['username'])){
    $admin = $_POST['username'];
    $password = $_POST['password'];
    $admin = stripslashes($admin);
    $password = stripslashes($password);
    $admin = strip_tags($admin);
    $password = strip_tags($password);

    if((!$admin) || (!$password)){
        $msg = "Invalid UserName or Password!!";
    } else {
        $admin = mysql_real_escape_string($admin);
        $password = md5($password);
        include_once("scripts/connect.php");
        $sql = mysql_query("SELECT * FROM admin where name='$admin' AND password='$password' LIMIT 1");
        $count = mysql_num_rows($sql);
        if($count>0){
            while($row = mysql_fetch_array($sql)){
                $id = $row['id'];
                $name = $row['name'];
                $pass = $row['password'];
                $_SESSION['id'] = $id;
                $_SESSION['name'] = $name;
                $_SESSION['password'] = $pass;
                mysql_query("UPDATE admin SET last_log=now() where name='$name' LIMIT 1");
                header("admin/index.php");  
            }
        } else {
            $msg = "Invalid UserName or Password!!";
        }
    }
}
?>

表单元素在这里：

<form action="admin.php" method="post" enctype="multipart/form-data">
            <table width="100%" cellpadding="5" cellspacing="5" border="0" >
                <tr>
                    <td colspan="2" align="center"><h1>Admin Login Page</h1></td>
                </tr>
                <tr>
                    <td colspan="2" align="center"><span style='font-weight:bold;color:red;'><?php echo $msg;?></span></td>
                </tr>
                <tr>
                    <td align="right"><label class="username_password">User Name:</label></td>
                    <td align="left"><input type="text" name="username" required placeholder="Enter username..." class="text-input" /></td>
                </tr>
                <tr>
                    <td align="right"><label class="username_password">Password:</label></td>
                    <td align="left"><input type="password" name="password" required placeholder="Enter password..." class="text-input" /></td>
                    </td>
                </tr>
                <tr>
                    <td colspan="2" align="center"><input type="submit" name="submit" id="submit" class="btnsubmit" value="Log In" /></td>
                </tr>
            </table>
        </form>

admin表的列名是（id，name，password，last_log）;

我该如何解决问题？