add sample app of signing in with google

README.md

-# sample-oauth2-client
-Sample OAuth2 client using the GitHub API
+# Sample OAuth 2.0 Clients
+
+Sample OAuth 2.0 clients using the GitHub and Google APIs
+
+Read more info in the book [https://oauth2simplified.com OAuth 2.0 Simplified]

google.php

+<?php
+// Fill these out with the values you got from Google
+$googleClientID = '';
+$googleClientSecret = '';
+
+// This is the URL we'll send the user to first to get their authorization
+$authorizeURL = 'https://accounts.google.com/o/oauth2/v2/auth';
+
+// This is Google's OpenID Connect token endpoint
+$tokenURL = 'https://www.googleapis.com/oauth2/v4/token';
+
+// The URL for this script, used as the redirect URL
+$baseURL = 'https://' . $_SERVER['SERVER_NAME'] . $_SERVER['PHP_SELF'];
+
+// Start a session so we have a place to store things between redirects
+session_start();
+
+
+
+// Start the login process by sending the user
+// to Google's authorization page
+if(isset($_GET['action']) && $_GET['action'] == 'login') {
+  unset($_SESSION['user_id']);
+
+  // Generate a random hash and store in the session
+  $_SESSION['state'] = bin2hex(random_bytes(16));
+
+  $params = array(
+    'response_type' => 'code',
+    'client_id' => $googleClientID,
+    'redirect_uri' => $baseURL,
+    'scope' => 'openid email https://www.googleapis.com/auth/userinfo.profile',
+    'state' => $_SESSION['state']
+  );
+
+  // Redirect the user to Google's authorization page
+  header('Location: ' . $authorizeURL . '?' . http_build_query($params));
+  die();
+}
+
+if(isset($_GET['action']) && $_GET['action'] == 'logout') {
+  unset($_SESSION['user_id']);
+  header('Location: '.$baseURL);
+  die();
+}
+
+// When Google redirects the user back here, there will be a "code" and "state"
+// parameter in the query string
+if(isset($_GET['code'])) {
+  // Verify the state matches our stored state
+  if(!isset($_GET['state']) || $_SESSION['state'] != $_GET['state']) {
+    header('Location: ' . $baseURL . '?error=invalid_state');
+    die();
+  }
+
+  // Exchange the auth code for a token
+  $ch = curl_init($tokenURL);
+  curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
+  curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query([
+    'grant_type' => 'authorization_code',
+    'client_id' => $googleClientID,
+    'client_secret' => $googleClientSecret,
+    'redirect_uri' => $baseURL,
+    'code' => $_GET['code']
+  ]));
+  $response = curl_exec($ch);
+  $data = json_decode($response, true);
+
+  // Note: You'd probably want to use a real JWT library
+  // but this will do in a pinch. This is only safe to do
+  // because the ID token came from the https connection
+  // from Google rather than an untrusted browser redirect
+
+  // Split the JWT string into three parts
+  $jwt = explode('.', $data['id_token']);
+
+  // Extract the middle part, base64 decode it, then json_decode it
+  $userinfo = json_decode(base64_decode($jwt[1]), true);
+
+  $_SESSION['user_id'] = $userinfo['sub'];
+  $_SESSION['email'] = $userinfo['email'];
+
+  // While we're at it, let's store the access token and id token
+  // so we can use them later
+  $_SESSION['access_token'] = $data['access_token'];
+  $_SESSION['id_token'] = $data['id_token'];
+  $_SESSION['userinfo'] = $userinfo;
+
+  header('Location: ' . $baseURL);
+  die();
+}
+
+
+
+// If there is a user ID in the session
+// the user is already logged in
+if(!isset($_GET['action'])) {
+  if(!empty($_SESSION['user_id'])) {
+    echo '<h3>Logged In</h3>';
+    echo '<p>User ID: '.$_SESSION['user_id'].'</p>';
+    echo '<p>Email: '.$_SESSION['email'].'</p>';
+    echo '<p><a href="?action=logout">Log Out</a></p>';
+
+    echo '<h3>ID Token</h3>';
+    echo '<pre>';
+    print_r($_SESSION['userinfo']);
+    echo '</pre>';
+
+    echo '<h3>User Info</h3>';
+    echo '<pre>';
+    $ch = curl_init('https://www.googleapis.com/oauth2/v3/userinfo');
+    curl_setopt($ch, CURLOPT_HTTPHEADER, [
+      'Authorization: Bearer '.$_SESSION['access_token']
+    ]);
+    curl_exec($ch);
+    echo '</pre>';
+
+  } else {
+    echo '<h3>Not logged in</h3>';
+    echo '<p><a href="?action=login">Log In</a></p>';
+  }
+  die();
+}
+