wordpress add sitemap.xml to robots.txt dynamically
/**
* Add Yoast SEO sitemap to virtual robots.txt file network wide
* https://wphelper.site/robots-txt-yoast-sitemap-multisite/
*/
function wphelper_network_robotstxt_function( $output, $public ) {
$homeURL = get_home_url();
$output .= "Sitemap: $homeURL/sitemap_index.xmln";
return $output;
}
add_filter( 'robots_txt', 'wphelper_network_robotstxt_function', 20, 2 );