function authenticate( $user_id, $user_name, $user_pass ) {
    global $current_user;
    $current_user = WP_User( $user_id );
 
    $return = wp_login( $user_name, $user_pass );
    if ( $return )
        $return = wp_set_cookie( $user_name, $user_pass );
    else
        return new WP_Error( 'Something went wrong while logging in!' );
 
    if ( $return )
        return true;
    else
        return new WP_Error( 'Something went wrong while setting a cookie!' );
}