PHP未定义变量注意，为什么我在这里得到它 - php undefined variable notice, why am i getting it here?

php undefined variable notice, why am i getting it here?

我阅读了关于使用三元运算符为未定义变量设置初始默认值的文章。所以这就是我在我的脚本顶部所做的，正如你将看到的。

然而，我仍然得到未定义的变量错误在我的联系人表单的输入值的字段。

有谁能解释为什么吗?

我代码:

<?php 
  if (isset($_POST["submit"])) {
    $user_name = isset($_POST["name"]) ? $_POST["name"] : "";
    $user_email = isset($_POST["email"]) ? $_POST["email"] : "";
    $user_age = isset($_POST["age"]) ? $_POST["age"] : "";
    $validation_errors = "";
    $error_counter = null;

    if (empty($_POST["name"])) {
      $validation_errors .= "<li class='validation_errors'>Please input your name.</li>";
      $error_counter++;
    }
    if (empty($user_email)) {
      $validation_errors .= "<li class='validation_errors'>Please input your email.</li>";
      $error_counter++;
    }
    if (empty($user_age)) {
      $validation_errors .= "<li class='validation_errors'>Please input your age.</li>";
      $error_counter++;
    } elseif (intval($user_age) < 18) {
      $validation_errors .= "<li class='validation_errors'>You must be over 18 years old.</li>";
      $error_counter++;
    }

    if ($error_counter != 0) {
      // $validation_errors = "<li class='validation_success'>Message successfully sent!</li>";
      $error_design = "<hr> <p id='error_msg'>Please Fix Below Errors: <p>";
    };
  }
?>

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>SandBox</title>
  <link rel="stylesheet" type="text/css" href="css/style.css">
  <link rel="stylesheet" href="css/animate.css">
  <script type="text/javascript" src="js/jquery.js"></script>
  <script type="text/javascript" src="js/script.js"></script>
</head>
<body>
  <header>
  </header>
  <form action="" method="post">
    <div class="input_group">
      <label for="name">Name</label>
      <input type="text" name="name" value="<?php echo htmlspecialchars($user_name); ?>">
    </div>
    <div class="input_group">
      <label for="email">eMail</label>
      <input type="email" name="email" value="<?php echo htmlspecialchars($user_email); ?>">
    </div>
    <div class="input_group">
      <label for="age">Age</label>
      <input type="text" name="age" value="<?php echo htmlspecialchars($user_age); ?>">
    </div>
    <input type="submit" value="Send" name="submit" id="submit">
    <?php echo $error_design; ?>
    <div>
      <ul>
        <?php echo $validation_errors; ?>
      </ul>
    </div>
  </form>

  <footer></footer>
</body>
</html>

将这些变量放在IF语句之前:

$user_name = isset($_POST["name"]) ? $_POST["name"] : "";
$user_email = isset($_POST["email"]) ? $_POST["email"] : "";
$user_age = isset($_POST["age"]) ? $_POST["age"] : "";

或者直接使用:

// Report all errors except E_NOTICE
error_reporting(E_ALL & ~E_NOTICE);