function set_affiliate_cookie() {

    if ( isset( $_GET['affiliate'] ) ) {
        setcookie( 'is_aff', intval( $_GET['affiliate'] ), time() + ( 30 * DAY_IN_SECONDS ), COOKIEPATH, 'thesacredscience.com' );
    }
}

add_action( 'init', 'set_affiliate_cookie', 20 );