I use that code in few Wordpress sites version 4.x. But it does not work with Wordpress 5. When I try to paste it in the functions.php file, I can't save it. Wordpress answers me 'Something goes wrong'.
add_action( "save_post", "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 .= "\n" . '<urlset xmlns=".9">' . "\n";
$sitemap .= "\t" . '<url>' . "\n" .
"\t\t" . '<loc>' . esc_url( home_url( '/' ) ) . '</loc>' .
"\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" . '</url>' . "\n";
}
$sitemap .= '</urlset>';
$fp = fopen( ABSPATH . "sitemap.xml", 'w' );
fwrite( $fp, $sitemap );
fclose( $fp );
}