Using OAuth 2.0 for Google Client-side Web Applications

http://pritomkumar.blogspot.com/2016/11/php-send-email-using-google-oauth2.html

1. Obtain OAuth 2.0 credentials from the Google API Console.
First visit to https://console.developers.google.com/ and follow the steps:
Click the "Credentials" as below image:

Now click on "Create credentials" right most part of button and then you can see a dropdown menu as below image:

Now click on "OAuth client ID"
It will show a page like and select "Web application" as below image and provide "Name" and "Authorized redirect URIs" as described and click "Create":

NB: it can say you to create a project, then create a project by click the button.

It will take you to the OAuth client page where you can see "Client ID" & "Client Secret" which would be need later.

You can enable any of your service from google account from list below for specific project:

All google products listed here:
https://developers.google.com/products/

2. Now its time to connect to google via OAuth (Its PHP code snippet):
You have to define your scope first:
$scope = "https://www.googleapis.com/auth/userinfo.profile%20https://www.googleapis.com/auth/userinfo.email%20https://www.googleapis.com/auth/gmail.readonly";
$auth_url = "https://accounts.google.com/o/oauth2/v2/auth";
$client_id = "YOUR CLIENT ID FROM GOOGLE";
$redirect_uri = "http://localhost/tappi/";

$forward_url = $auth_url."?scope=".$scope."&redirect_uri=".urlencode($redirect_uri)."&response_type=code&client_id=".urlencode($client_id);
header("Location: ".$url);

It will redirect you to the following page (if not logged in, login then):

Now click "Allow" button direct you to url you provided when creating application with a code in get parameter
Now you can get access token using the code received.
Full example code below:

<?php
session_start();
init();

if(token() != null) {
    echo "<a href='".$_SESSION["redirect_uri"]."'>Home</a>";
    echo " || <a href='".$_SESSION["redirect_uri"]."?refresh_token=true'>Refresh token</a>";
    echo " || <a href='".$_SESSION["redirect_uri"]."?profile=true'>Profile</a>";
    echo " || <a href='".$_SESSION["redirect_uri"]."?logout=true'>Logout</a><br/><br/>\n\n";
}

if(isset($_GET["logout"])) {
    flushToken();
    echo "Logged out<br/>";
    echo "<a href='".$_SESSION["redirect_uri"]."'>Start new session</a>";
    die();
}
else if(isset($_GET["refresh_token"])) {
    refreshToken();
    header("Location: ".$_SESSION["redirect_uri"]);
}
else if(isset($_GET["profile"])) {
    viewProfile();
}
else if(token() != null) {
    echo "<pre>";print_r(token());echo "</pre>";
}
else if(isset($_GET["code"])) {
    $post = "code=".urlencode($_GET["code"])."&client_id=".urlencode($_SESSION["client_id"]);
    $post .= "&client_secret=".urlencode($_SESSION["client_secret"]);
    $post .="&redirect_uri=".urlencode($_SESSION["redirect_uri"]);
    $post .= "&grant_type=authorization_code";
    $result = json_decode(runCurl($_SESSION["token_url"], $post));
    storeToken($result);
    if(isset($result->refresh_token)) {
        file_put_contents("rt-".getUserID().".txt", $result->refresh_token);
    }
    file_put_contents("active.txt", getUserID());
    file_put_contents("access_token.txt", $result->access_token);
    header("Location: " . $_SESSION["redirect_uri"]);
}
else {
    $url = $_SESSION["auth_url"]."?scope=".urlencode($_SESSION["scope"]).
        "&redirect_uri=".urlencode($_SESSION["redirect_uri"]).
        "&response_type=code&client_id=".urlencode($_SESSION["client_id"])."&access_type=offline";
    echo "<a href='".$url."'>Authorize with Google</a>";
}

function refreshToken() {
    $post = "client_id=".urlencode($_SESSION["client_id"]);
    $post .= "&client_secret=".urlencode($_SESSION["client_secret"]);
    $post .= "&redirect_uri=".urlencode($_SESSION["redirect_uri"]);
    $post .= "&grant_type=refresh_token&refresh_token=".urlencode(getRefreshToken());
    $result = json_decode(runCurl($_SESSION["token_url"], $post));
    file_put_contents("access_token.txt", $result->access_token);
    storeToken($result);
}

function getRefreshToken() {
    $active = file_get_contents("active.txt");
    return file_get_contents("rt-".$active.".txt");
}

function flushToken() {
    file_put_contents("auth.txt", "");
    $_SESSION["redirected"] = null;
}

function token() {
    $text = file_exists("auth.txt") ? file_get_contents("auth.txt") : null;
    if($text != null && strlen($text) > 0) {
        return json_decode($text);
    }
    return null;
}

function storeToken($o) {
    file_put_contents("auth.txt", json_encode($o));
}

function init() {
    $_SESSION["auth_url"] = "https://accounts.google.com/o/oauth2/v2/auth";
    $_SESSION["token_url"] = "https://accounts.google.com/o/oauth2/token";
    $_SESSION["client_id"] = "892386593019-xxxxxxxxxinht701m7kn0gkoj964r2.apps.googleusercontent.com";
    $_SESSION["client_secret"] = "bVQ_xT0ZxxxxxxxxxxvV9zRV3";
    $_SESSION["redirect_uri"] = "http://localhost/tappi/google.php";
    $_SESSION["scope"] = "https://www.googleapis.com/auth/userinfo.profile"; /* User profile */
    $_SESSION["scope"] .= " https://www.googleapis.com/auth/userinfo.email"; /* User email address */
    $_SESSION["scope"] .= " https://www.googleapis.com/auth/gmail.readonly"; /* Read mail */
    $_SESSION["scope"] .= " https://www.googleapis.com/auth/gmail.send"; /* Send email */
}

function getUserID() {
    $fromSession = valueFromSession("google_user_id");
    if($fromSession) {
        return $fromSession;
    }
    else {
        $apiUrl = "https://www.googleapis.com/oauth2/v1/userinfo?alt=json";
        $apiUrl .= "&access_token=".token()->access_token;
        $result = json_decode(runCurl($apiUrl));
        $_SESSION["google_user_id"] = $result->id;
        return $_SESSION["google_user_id"];
    }
}

function valueFromSession($name) {
    if(isset($_SESSION[$name])) {
        return $_SESSION[$name];
    }
    return null;
}

function viewProfile() {
    $apiUrl = "https://www.googleapis.com/oauth2/v1/userinfo?alt=json";
    $apiUrl .= "&access_token=".token()->access_token;
    $result = json_decode(runCurl($apiUrl));
    echo "<pre>";
    print_r($result);
    echo "</pre>";
}

function runCurl($url, $post = null, $headers = null) {
    $curl = curl_init();
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_POST, $post == null ? 0 : 1);
    if($post != null) {
        curl_setopt($curl, CURLOPT_POSTFIELDS, $post);
    }
    curl_setopt($curl, CURLOPT_HTTPAUTH, CURLAUTH_ANY);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($curl, CURLOPT_SSLVERSION, 1);
    if($headers != null) {
        curl_setopt($curl, CURLOPT_HEADER, true);
        curl_setopt($curl, CURLOPT_HTTPHEADER, $headers);
    }
    $response = curl_exec($curl);
    $http_code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
    curl_close($curl);
    if($http_code >= 400) {
        echo "Error executing request to Office365 api with error code=$http_code<br/><br/>\n\n";
        echo "<pre>"; print_r($response); echo "</pre>";
        die();
    }
    return $response;
}
?>