[Add sitemap to the Robots file] Add the sitemap to the robots.txt file for SEO #wordpress

robots_txt.php

add_action('plugins_loaded', function(){	
	add_filter( 'robots_txt', 'maybe_modify_robots_txt', 20, 2 );	
}, 999);

/**
	  * Modify the robots.txt, if not present
	  *
	  * @param string $output The robots.txt output 
	  * @param boolean Whether the site is considered "public".
	  *
	  * @return string Robots.txt output
	  */
	  
	public function maybe_modify_robots_txt($output, $public)
	{
		// Don't add sitemap to robots.txt if website is in development
		if (get_option( 'blog_public' ) === '1') {
		
			// First add sitemap if Yoast is active
	
			if (self::yoast_is_active()) {
				
				if (function_exists('get_sites')) {
					
					foreach (get_sites() AS $site) {
						
						if ($site->public == 1 && $site->deleted == 0) {
							$output .= "\nSitemap: ".get_site_url($site->blog_id)."/sitemap_index.xml";
						}
						
					}
					
				} else {
					
					$output .= "\nSitemap: ".home_url()."/sitemap_index.xml";
					
				}
				
				
			
			}
		
		}
		
      //todo: add custom robots lines
		$robots_lines = "";
		
		if (!empty($robots_lines)) {
			$output .= "\n\n".$robots_lines;
		}

		
		// Check of custom rules
		
		return $output;
		
	}