Send Feed Readers to Your Feed Service

// Your feed's address at the feed service (FeedPress, Follow.it, Feedburner ...). Leave empty to turn the redirect off.
$wpide_feed_service_url = '';
// Parts of the service's user agent; these requests get your real feed so the service can read it.
$wpide_feed_service_agents = array( 'feedpress', 'feedburner', 'follow.it', 'feedvalidator' );

add_action( 'template_redirect', function () use ( $wpide_feed_service_url, $wpide_feed_service_agents ) {
    if ( '' === $wpide_feed_service_url || ! is_feed() || is_comment_feed() || is_archive() || is_search() || is_singular() ) {
        return;
    }

    // The service can also read the original feed at /feed/?origin-feed=1.
    if ( isset( $_GET['origin-feed'] ) ) {
        return;
    }

    $agent = isset( $_SERVER['HTTP_USER_AGENT'] ) ? strtolower( sanitize_text_field( wp_unslash( $_SERVER['HTTP_USER_AGENT'] ) ) ) : '';

    foreach ( $wpide_feed_service_agents as $needle ) {
        if ( '' !== $agent && false !== strpos( $agent, strtolower( $needle ) ) ) {
            return;
        }
    }

    // 302 keeps subscribers on your own feed address, so you can switch services later.
    wp_redirect( esc_url_raw( $wpide_feed_service_url ), 302 ); // phpcs:ignore WordPress.Security.SafeRedirect -- external address set by the site owner.
    exit;
}, 1 );