XML sitemap in Wordpress 5 without plugin?

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 );
}