callback.php

<?php
require_once __DIR__ . '/vendor/autoload.php';

$dotenv = new Dotenv\Dotenv(__DIR__);
$dotenv->load();

$provider = new \League\OAuth2\Client\Provider\GenericProvider([
    'clientId'                => getenv('CLIENT_ID'),
    'clientSecret'            => getenv('CLIENT_SECRET'),
    'redirectUri'             => getenv('REDIRECT_URI'),
    'urlAuthorize'            => getenv('AUTH_URL'),
    'urlAccessToken'          => getenv('TOKEN_URL'),
    'urlResourceOwnerDetails' => getenv('ATTRIBUTES_URL')
]);

session_start();

if (!isset($_GET['code'])) {
    // Fetch the authorization URL from the provider; this returns the
    // urlAuthorize option and generates and applies any necessary parameters
    // (e.g. state).
    $authorizationUrl = $provider->getAuthorizationUrl();

    // Get the state generated for you and store it to the session.
    $_SESSION['oauth2state'] = $provider->getState();

    // Redirect the user to the authorization URL.
    header('Location: ' . filter_var($authorizationUrl, FILTER_SANITIZE_URL));
    exit;

} else {
    try {
        // Try to get an access token using the authorization code grant.
        // Pass in the affiliation in the scope
        $accessToken = $provider->getAccessToken('authorization_code', [
            'code' => $_GET['code'], 'scope' => 'military'
        ]);

        // // Using the access token, we may look up details about the user
        $resourceOwner = $provider->getResourceOwner($accessToken);

        // // The JSON payload of the response
        $_SESSION['payload'] = json_encode($resourceOwner->toArray());

        header('Location: ' . '/');
        exit;

    } catch (\League\OAuth2\Client\Provider\Exception\IdentityProviderException $e) {

        // Failed to get the access token or user details.
        exit($e->getMessage());
    }
}