Recent Posts From Across the Network

if ( ! is_multisite() ) {
    return;
}

add_shortcode( 'wpide_network_posts', function ( $atts ) {
    $atts  = shortcode_atts( array( 'number' => 10, 'sites' => '' ), $atts, 'wpide_network_posts' );
    $limit = min( 30, max( 1, absint( $atts['number'] ) ) );
    $ids   = array_filter( array_map( 'absint', explode( ',', $atts['sites'] ) ) );
    $key   = 'wpide_network_posts_' . md5( $limit . '|' . implode( ',', $ids ) );
    $items = get_site_transient( $key );

    if ( false === $items ) {
        $items = array();
        $sites = get_sites( array(
            'site__in' => $ids,
            'public'   => 1,
            'archived' => 0,
            'spam'     => 0,
            'deleted'  => 0,
            'number'   => 50,
            'orderby'  => 'last_updated',
            'order'    => 'DESC',
            'fields'   => 'ids',
        ) );

        foreach ( $sites as $blog_id ) {
            switch_to_blog( $blog_id );
            $name = get_bloginfo( 'name' );
            foreach ( get_posts( array( 'numberposts' => $limit, 'no_found_rows' => true ) ) as $post ) {
                $items[] = array(
                    'time'  => strtotime( $post->post_date_gmt ),
                    'title' => get_the_title( $post ),
                    'url'   => get_permalink( $post ),
                    'site'  => $name,
                );
            }
            restore_current_blog();
        }

        usort( $items, function ( $a, $b ) {
            return $b['time'] <=> $a['time'];
        } );
        $items = array_slice( $items, 0, $limit );
        set_site_transient( $key, $items, 30 * MINUTE_IN_SECONDS );
    }

    if ( ! $items ) {
        return '';
    }

    $html = '<ul class="wpide-network-posts">';
    foreach ( $items as $item ) {
        $html .= sprintf(
            '<li><a href="%1$s">%2$s</a> <small>%3$s · %4$s</small></li>',
            esc_url( $item['url'] ),
            esc_html( $item['title'] ),
            esc_html( $item['site'] ),
            esc_html( wp_date( get_option( 'date_format' ), $item['time'] ) )
        );
    }

    return $html . '</ul>';
} );