<?php 
/* function to create sitemap.xml file in root directory of site  */
// add_action("publish_post", "eg_create_sitemap");
// add_action("publish_page", "eg_create_sitemap");
add_action( "save_post", "eg_create_sitemap" );
function eg_create_sitemap() {
    if ( str_replace( '-', '', get_option( 'gmt_offset' ) ) < 10 ) { 
        $tempo = '-0' . str_replace( '-', '', get_option( 'gmt_offset' ) ); 
    } else { 
        $tempo = get_option( 'gmt_offset' ); 
    }
    if( strlen( $tempo ) == 3 ) { $tempo = $tempo . ':00'; }
    $postsForSitemap = get_posts( array(
        'numberposts' => -1,
        'orderby'     => 'modified',
        'post_type'   => array( 'post', 'page' ),
        'order'       => 'DESC'
    ) );
    $sitemap .= '<?xml version="1.0" encoding="UTF-8"?>' . '<?xml-stylesheet type="text/xsl" href="' . 
        esc_url( home_url( '/' ) ) . 'sitemap.xsl"?>';
    $sitemap .= "\n" . '<urlset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd" xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">' . "\n";
    $sitemap .= "\t" . '<url>' . "\n" .
        "\t\t" . '<loc>' . esc_url( home_url( '/' ) ) . '</loc>' .
        "\n\t\t" . '<lastmod>' . date( "Y-m-d\TH:i:s", current_time( 'timestamp', 0 ) ) . $tempo . '</lastmod>' .
        "\n\t\t" . '<changefreq>daily</changefreq>' .
        "\n\t\t" . '<priority>1.0</priority>' .
        "\n\t" . '</url>' . "\n";
    foreach( $postsForSitemap as $post ) {
        setup_postdata( $post);
        $postdate = explode( " ", $post->post_modified );
        $sitemap .= "\t" . '<url>' . "\n" .
            "\t\t" . '<loc>' . get_permalink( $post->ID ) . '</loc>' .
            "\n\t\t" . '<lastmod>' . $postdate[0] . 'T' . $postdate[1] . $tempo . '</lastmod>' .
            "\n\t\t" . '<changefreq>Weekly</changefreq>' .
            "\n\t\t" . '<priority>0.5</priority>' .
            "\n\t" . '</url>' . "\n";
    }
    $sitemap .= '</urlset>';
    $fp = fopen( ABSPATH . "sitemap.xml", 'w' );
    fwrite( $fp, $sitemap );
    fclose( $fp );
}