注意:未定义的索引:用户名在C:xampphtdocsindex.php在第4行 - Notice: Undefined index: username in C:xampphtdocsindex.php on line 4

Notice: Undefined index: username in C:xampphtdocsindex.php on line 4

大家好首先我要感谢你的时间，我必须为大学做这个(web服务器脚本单元p4)这是我的简单登录，但不能让它运行并不断得到错误信息:注意:未定义的索引:用户名在C:'xampp'htdocs'index.php在第4行

下面是我的代码:

<?php
session_start();
if($_POST["username"]) {
    $dbUsname = "Stephen";
    $dbPassword = "Password123";
    $uid = "0001";
    $usname = strip_tags($_POST["username"]);
    $pass = strip_tags($_POST["password"]);
    if (username == $dbUsname && $pass == $dbPassword) {
        $_SESSION["username"] = $usname;
        $_SESSION["id"] = $uid;
        header("Location; user.php");
        } else {
            echo"<h2>UH OH Oh no you didnt your password was incorrect.
            <br> /> Enter that again.</h2>";
        }
}
?>
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Basic Login System</title>
<style type="text/css">
html {
    font-family: Verdano, Genevo, sans-serif;
}
h1{
    font-size: 24px;
    text-align: center;
}
#wrapper {
    position: absolute;
    width: 100px;
    top: 30%;
    margin-top: -50%;
}
#form {
    margin: auto;
    width: 200px;
    height: 100px
}
</style>
</head>
<body>
<div id="wrapper">
<h1>Login</h1>
<form id="form" action="index.php" method="post" enctype="multiport/form-data">
Username: <input type="text" name="username" /> <br> />
Password: <input type="password" name="password" /> />
<input type="submit" value="Login" name="Submit" />
</form>
</body>
</html>

$_POST可能没有任何名为"name"的索引。使用isset($_POST["name"])检查索引是否存在