php怎么实现app的验证登录？（代码示例）

<?php
$servername = "localhost";
$username = "username";
$passWord = "password";

try {
  $conn = new PDO("mysql:host=$servername;dbname=myDB", $username, $password);
  $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  echo "Connected successfully";
} catch(PDOException $e) {
  echo "Connection failed: " . $e->getMessage();
}
?>

<?php
// 验证用户输入数据
if ($_SERVER["REQUEST_METHOD"] == "POST") {
  // 获取POST请求中的数据
  $username = $_POST["username"];
  $password = $_POST["password"];

  // 以预编译语句方式查询数据库中用户信息
  $stmt = $conn->prepare("SELECT * FROM users WHERE username=:username AND password=:password");
  $stmt->bindParam(':username', $username);
  $stmt->bindParam(':password', $password);
  $stmt->execute();

  $result = $stmt->setFetchMode(PDO::FETCH_ASSOC);
  $rows = $stmt->fetchAll();

  if (count($rows) > 0) {
    // 用户验证成功
    echo "Login successfully";
    // 将session或token等信息返回给移动端APP
    // 略
  } else {
    // 用户验证失败
    echo "Login failed";
  }
}
?>

<?php
// 请求验证处理
if ($_SERVER["REQUEST_METHOD"] == "GET") {
  // 从请求头中获取session或token信息
  $token = $_SERVER['HTTP_TOKEN'];

  // 判断session或token是否存在或已失效
  if (isset($_SESSION['token']) && $_SESSION['token'] == $token) {
    // 请求验证成功
    echo "Request authorized";
    // 略
  } else {
    // 请求验证失败
    echo "Request unauthorized";
  }
}
?>