<?php 
 
// Create sitemap.xml with each publish
// Original http://emrahgunduz.com/category/categories/development/wordpress/
 
add_action("publish_post", "eg_create_sitemap"); 
add_action("publish_page", "eg_create_sitemap"); 
 
function eg_create_sitemap() {   
  $postsForSitemap = get_posts(array(
        'numberposts' => -1,
        'orderby' => 'modified',
        'post_type'  => array('post','page'),
        'order'    => 'DESC'
   ));
     $sitemap = '<?xml version="1.0" encoding="UTF-8"?>';   
     $sitemap .= '<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">';     
 
     foreach($postsForSitemap as $post) {     
      setup_postdata($post);         
 
      $postdate = explode(" ", $post->post_modified);         
 
      $sitemap .= '<url>'.       
        '<loc>'. get_permalink($post->ID) .'</loc>'.       
        '<lastmod>'. $postdate[0] .'</lastmod>'.       
        '<changefreq>monthly</changefreq>'.     
        '</url>';   
        }     
 
    $sitemap .= '</urlset>';     
 
    $fp = fopen(ABSPATH . "sitemap.xml", 'w');   
    fwrite($fp, $sitemap);   
    fclose($fp); 
 
} 
 
?>