未定义的索引:C:xampphtdocsform_require1.php 第 53/55 行中的名字

大家好！ 有人可以指导我解决以下问题吗？我试图制作一个 php 表单，但它在我运行时显示错误（未定义的索引：C：''xampp''htdocs''form_require1.php 第 53/55 行中的名字）。请帮助我

enter code here    <?php // define valiables and set to empty values $firstnameErr = $lastnameErr = ""; $firstname = $lastname = ""; if ($_SERVER['REQUEST_METHOD'] == "POST") {
if (empty($_POST["firstname"])) {
    $firstnameErr = "Name is required";
  } else {
    $firstname = test_input($_POST["firstname"]); }
 if (empty($_POST["lastname"])){
$lastnameErr = "Name is require"; } else { $lastname = test_input($_POST["lastname"]);  } } function test_input($data) {
$data = trim($data);
$data = stripslashes($data);
return $data; } ?>    `enter code here`    <form action="<?php echo htmlentities($_SERVER["PHP_SELF"]) ?>" method="post"> Your firstname <input type="text" name="firstname" /> <span class="error">* <?php echo $firstnameErr;?></span> <br><br> Your Lastname <input type="text" name="lastname" /> <span class="error">* <?php echo $lastnameErr;?></span> <br><br> <input type="submit" value="Submit" /> </form> <?php  // } else {
echo 'Your Details'; 
echo "<br>";
echo  'Fistname: ' . $_POST["firstname"];
echo "<br>";
echo  'Lastname: ' . $_POST["lastname"];