源代码:

<?php
// 定义变量并默认设为空值
$nameErr = $emailErr = $genderErr = $websiteErr = "";
$name = $email = $gender = $comment = $website = "";

if ($_SERVER["REQUEST_METHOD"] == "POST") {
   if (empty($_POST["name"])) {
      $nameErr = "名字是必须的。";
   } else {
      $name = test_input($_POST["name"]);
   }

if (empty($_POST["email"])) {
      $emailErr = "邮箱是必须的。";
   } else {
      $email = test_input($_POST["email"]);
   }

if (empty($_POST["website"])) {
      $website = "";
   } else {
      $website = test_input($_POST["website"]);
   }

if (empty($_POST["comment"])) {
      $comment = "";
   } else {
      $comment = test_input($_POST["comment"]);
   }

if (empty($_POST["gender"])) {
      $genderErr = "性别是必须的。";
   } else {
      $gender = test_input($_POST["gender"]);
   }
}

function test_input($data) {
   $data = trim($data);
   $data = stripslashes($data);
   $data = htmlspecialchars($data);
   return $data;
}
?>

<h2>PHP 表单验证实例</h2>
<p><span class="error">* 必填字段。</span></p>
<form method="post" action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>"> 
   名字: <input type="text" name="name">
   <span class="error">* <?php echo $nameErr;?></span>

E-mail: <input type="text" name="email">
   <span class="error">* <?php echo $emailErr;?></span>

网址: <input type="text" name="website">
   <span class="error"><?php echo $websiteErr;?></span>

备注: <textarea name="comment" rows="5" cols="40"></textarea>

性别:
   <input type="radio" name="gender" value="female">女
   <input type="radio" name="gender" value="male">男
   <span class="error">* <?php echo $genderErr;?></span>

<?php
echo "<h2>您的输入:</h2>";
echo $name;
echo "
";
echo $email;
echo "
";
echo $website;
echo "
";
echo $comment;
echo "
";
echo $gender;
?>

源代码:

运行结果：