session tokens not set

[str]

In the config we see:

$config = new \Upwork\API\Config(
    array(
        'consumerKey'       => 'xxxxxxxxxxxxxxxxxxxxxxxxxxxx',  // SETUP YOUR CONSUMER KEY
        'consumerSecret'    => 'xxxxxxxxxxxxx',                  // SETUP YOUR KEY SECRET
        'accessToken'       => $_SESSION['access_token'],       // got access token
        'accessSecret'      => $_SESSION['access_secret'],      // got access secret
        'requestToken'      => $_SESSION['request_token'],      // got request token
        'requestSecret'     => $_SESSION['request_secret'],     // got request secret
        'verifier'          => $_GET['oauth_verifier'],         // got oauth verifier after authorization
        'mode'              => 'web',                           // can be 'nonweb' for console apps (default),
                                                                // and 'web' for web-based apps
//  'debug' => true, // enables debug mode. Note that enabling debug in web-based applications can block redirects
//  'authType' => 'MyOAuth' // your own authentication type, see AuthTypes directory
    )
);

but $_SESSION['access_token'] nor $_SESSION['access_secret'] are set yet,