SitemapHelper.php

<?php
namespace backend\helpers;

use Yii;
use yii\db\Query;
use yii\base\Object;
use SitemapPHP\Sitemap;
/**
 * 
 * This is based on https://github.com/evert/sitemap-php/
 * 
 **/

class SitemapHelper extends Object
{
    protected static function getSlug($string)
    {
        $hyphened = preg_replace('/\s+/', '-', trim($string, " ")); // Replaces all spaces with hyphens.
        $alpha_num = preg_replace('/[^A-Za-z0-9\-]/', '', $hyphened); // stores only alphnumeric characters into $string
        $string = preg_replace('/[--]+/', '-', $alpha_num);
        return strtolower($string);
    }
    
    
    public static function updateSitemap(){
        $sitemap_path   =   '/path/to/public_html/sitemap.xml';        
        
        if (file_exists($sitemap_path)) {
            shell_exec('rm -rf '.$sitemap_path);
        }        
        
        $sitemap    =   new Sitemap('http://example.com');	
        $sitemap->setPath('/path/to/public_html/');
        $sitemap->setFilename('sitemap');    
        
        $query = (new Query)->from('page')->where(['status' => 1]);
        
        foreach ($query->each() as $page) {            
            $sitemap->addItem('/'.$page['url'], '0.8', 'daily', date('Y-m-d', $page['updated_at']));
        }       
        $sitemap->createSitemapIndex('http://example.com/', 'Today');  
    }
}