jam.php

<?php
function getUserInfos(
    $access_token,
    $jam = [
        'app_id' => '',
        'redirect_url' => '',
        'api_secret' => ''
    ]
) {
    $response = json_decode(file_get_contents(
        "https://core.justauth.me/api/data?secret=" . $jam['api_secret'] . "&access_token=$access_token",
        false,
        stream_context_create([
            'http' => [
                'ignore_errors' => true
            ]
        ])
    ), true);

    switch (substr($http_response_header[0], 9, 3)) {
        case 200:
            return $response;
            break;
        case 400:
            throw new \Exception(
                'JamBadRequestException:Access-Token and API Secret are required.
             Please contact support@justauth.me',
                400
            );
            break;

        case 401:
            throw new \Exception('JamUnauthorizedException:Api Secret is invalid', 401);
            break;

        case 404:
            throw new \Exception('JamNotFoundException:No such Access-Token', 404);
            break;

        case 500:
            throw new \Exception(
                'JamInternalServerErrorException:Wrong data format.
            Please contact support@justauth.me',
                500
            );
            break;

        default:
            throw new \Exception('JamUnknowErrorException:Unknow error. Please contact support@justauth.me');
    }
}

if (isset($_GET['access_token'])) {
    // The expected access_token is present
    try {
        $id = getUserInfos($_GET['access_token']);

        /*
         * Everything is fine, you can now register or login the user,
         * depending on the presence in your Database of
         * the provided $user_infos->jam_id
         */
    } catch (Exception $e) {
        die($e->getMessage());
        // Login fail, you should redirect to an error page
        header('Location: /index.php');
        exit;
    }
} else {
    /*
     * The callback URL wasn't called with the correct parameter
     * you should redirect to an error page
     */
    header('Location: /index.php');
    exit;
}