空白字段问题

我注意到，当我故意不输入所需字段来扰乱表单时，它会在第一次工作（即触发错误消息）。但是，当我尝试填写其余缺失的字段并点击提交时，表单再次显示为空白。

或者，如果我错过了一个字段，然后尝试输入缺失的字段（已触发错误），并在提交前故意将另一个字段留空，则表单再次显示为空白。为什么？

（我的PHP真的很糟糕。所以，请慢慢来。）

PHP表单

<?php
    session_start(); //allows use of session variables

    if ($_SERVER["REQUEST_METHOD"] == "POST" && $_SESSION['token'] == $_POST['token'] && !empty($_SESSION['token'])) {
       if (empty($_POST["first-name"])) {
         $firstNameErr = "First name is required";
       } else {
         $first_name = test_input($_POST["first-name"]);
       }
        if (empty($_POST["last-name"])) {
         $lastNameErr = "Last name is required";
       } else {
         $last_name = test_input($_POST["last-name"]);
       }
       if (empty($_POST["email"])) {
         $emailErr = "Email is required";
       } else {
         $email = test_input($_POST["email"]);
       }
       if (empty($_POST["message"])) {
         $messageErr = "Message is required";
       } else {
         $message = test_input($_POST["message"]);
       }

       if(isset($first_name) && isset($last_name) && isset($email) && isset($message))
       {
         $_SESSION['first_name'] = $first_name;
         $_SESSION['last_name'] = $last_name;
         $_SESSION['email'] = $email;
         $_SESSION['message'] = $message;
         header("Location: contact9SessionsCRSF2.php");
         exit; 
       }
    }
    else {
      $token = md5(uniqid(rand(), TRUE));
      $_SESSION['token'] = $token;
      $_SESSION['token_time'] = time();
    }
    function test_input($data) {
       $data = trim($data);
       $data = stripslashes($data);
       $data = htmlspecialchars($data);
       return $data;
    }
    ?>

HTML表单

  <form class="form-horizontal"  method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>">
  <input type="hidden" name="token" value="<?php echo $token; ?>" />
    <div class="form-group">
      <label class="col-sm-2 control-label">First Name</label>
      <div class="col-sm-10">
      <input class="form-control" name="first-name" id="first-name" placeholder="First Name" type="text" value="<?php if(isset($first_name)) { echo $first_name; }?>">
       <?php if(isset($firstNameErr)) print ('<span class="error">* ' . $firstNameErr . '</span>'); ?>
      </div>
    </div>
    <div class="form-group">
      <label class="col-sm-2 control-label">Last Name</label>
      <div class="col-sm-10">
      <input class="form-control" name="last-name" id="last-name" placeholder="Last Name" type="text" value="<?php if(isset($last_name)) { echo $last_name; }?>">
       <?php if(isset($lastNameErr)) print ('<span class="error">* ' . $lastNameErr . '</span>'); ?>
     </div>
    </div>
      <div class="form-group">
      <label class="col-sm-2 control-label">Email</label>
      <div class="col-sm-10">
      <input class="form-control" name="email" id="email" placeholder="Email" type="email" >
       <?php if(isset($emailErr)) print ('<span class="error">* ' . $emailErr . '</span>'); ?>
      </div>
    </div>
    <div class="form-group">
      <label class="col-sm-2 control-label">Message</label>
      <div class="col-sm-10">
      <textarea class="form-control" rows="2" placeholder="Please type in your message" name="message" id="message"></textarea>
       <?php if(isset($messageErr)) print ('<span class="error">* ' . $messageErr . '</span>'); ?>
      </div>
    </div>
    <div class="form-group">
    <div class="col-sm-offset-2 col-sm-10">
      <button type="submit" class="btn btn-default">Submit</button>
    </div>
  </div>
</form>