如何实现PHP表单提交后的数据验证

<form action="handle_form.php" method="post">
  <input type="text" name="username" placeholder="用户名">
  <input type="password" name="password" placeholder="密码">
  <input type="submit" value="提交">
</form>

<?php
$username = $_POST['username'];
$password = $_POST['password'];

// 验证用户名
if (empty($username)) {
  echo "用户名不能为空";
}

// 验证密码
if (empty($password)) {
  echo "密码不能为空";
}

// 验证邮箱
$email = $_POST['email'];
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
  echo "邮箱格式不正确";
}

// 验证URL
$url = $_POST['url'];
if (!filter_var($url, FILTER_VALIDATE_URL)) {
  echo "URL格式不正确";
}
?>

<?php
$errors = [];

// 验证用户名
if (empty($username)) {
  $errors['username'] = "用户名不能为空";
}

// 验证密码
if (empty($password)) {
  $errors['password'] = "密码不能为空";
}

// 验证邮箱
$email = $_POST['email'];
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
  $errors['email'] = "邮箱格式不正确";
}

// 验证URL
$url = $_POST['url'];
if (!filter_var($url, FILTER_VALIDATE_URL)) {
  $errors['url'] = "URL格式不正确";
}

if (!empty($errors)) {
  foreach ($errors as $error) {
    echo $error;
  }
}
?>

<?php
$username = $_POST['username'];
$username = mysqli_real_escape_string($conn, $username);
// ...
?>