PHP $_POST 获取未定义的索引，即使给出了输入 - PHP $_POST getting undefined index, even though inputs are given

>我正在尝试使用 $_POST 方法从 HTML 表单中获取用户名和密码，但是我在给定输入时遇到未定义的索引错误。

我的索引文件的表单部分如下

<form class="form-signin"  role="form"action="" method="post">
        <h2 class="form-signin-heading">title<em class='current'>title</em></h2>
        <input id="username" type="username" class="form-control" placeholder="Username" required autofocus>
        <input id="password" type="password" class="form-control" placeholder="Password" required>
        <input name ="submit" type="submit" value="sign in">
      </form>

我的 PHP 脚本（称为 auth.php）也包含在索引文件的顶部。

这是我的身份验证.php脚本代码示例

<?php
ini_set('display_errors',1);
error_reporting(E_ALL);
echo "auth is called";
if(isset($_POST['submit'])){
echo "submit is pressed";
//this if statement enters when submit is pressed
if(empty($_POST['username']) || empty($_POST['password'])) {
echo "Username or Password is empty";
//this if statement enters too, even with input given
}
else{
echo "YAY";
}
}
?>

"用户名或密码为空"语句不断被打印，即使我在输入字段中输入用户名和密码也是如此。问题出在哪里

缺少"name"attribute in input字段，它应该是

编辑2：用户名input中缺少type="text"

 <input type="text" name='username' id="username" class="form-control" placeholder="Username" required autofocus>
 <input name='password' id="password" type="password" class="form-control" placeholder="Password" required>

更正您的表单， 1.姓名缺失，2。没有用户名名称的数据类型

<form class="form-signin" role="form" action="" name="loginform" method="post">
    <h2 class="form-signin-heading">title<em class='current'>title</em></h2>
    <input id="username" name="username" type="text" class="form-control" placeholder="Username" required autofocus>
    <input id="password" name="password" type="password" class="form-control" placeholder="Password" required>
    <input name="submit" type="submit" value="sign in">
</form>

<input id="username" name="username" type="username" class="form-control" placeholder="Username" required autofocus>
            <input id="password" type="password" name="password" class="form-control" placeholder="Password" required>
    <?php
    ini_set('display_errors',1);
    error_reporting(E_ALL);
    echo "auth is called";
    if(isset($_POST['submit'])){
    echo "submit is pressed";
    //this if statement enters when submit is pressed
    if(empty($_POST['username']) || empty($_POST['password'])) {
    echo "Username or Password is empty";
    //this if statement enters too, even with input given
    }
    else if(empty($_POST['username']){
      echo "Username  is empty";
    }
    else if(empty($_POST['password']){
      echo "Password  is empty";
    }
    else{
    echo "YAY";
    }
    }
    ?>

输入type = text和name attr必需

 <input name='username' id="username" type="text" class="form-control" placeholder="Username" required autofocus>

如果未获得所需的输出，请使用 print_r() 或 var_dump 检查值。

这将帮助您检查代码的哪一部分出现故障。如果你这样做了，你会发现你的html表单内容只有问题，因为你不会打印任何字段内容，你可以很容易地把它整理出来......

将username输入的类型设置为 "text" ：

<input name='username' id="username" type="text" class="form-control" placeholder="Username" required autofocus>
<input name='password' id="password" type="password" class="form-control" placeholder="Password" required>