follow the guideline for generating sitemap XML file :
below is simple sitemap.xml file :
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://www.a.com/b.html</loc>
<lastmod>2021-06-04</lastmod>
</url>
</urlset>
i am considering you have all link in database which need to include in sitemap.xml
suppose sitemap_urls have filed url and page upadte time , you can manage it by admin
so following code is for generating xml sitemap in PHP
//database connection
$connection=mysqli_connect(...
..
..
$sitemapcontent='';
$sitemapcontent=$sitemapcontent.'<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">';
$result=mysqli_query("select * from sitemap_urls ",$connection);
while($siteslist=mysqli_fetch_arrray($result)){
$sitemapcontent=$sitemapcontent.'
<url>
<loc>'.$siteslist['url'].'</loc>
<lastmod>'.$siteslist['update_date'].'</lastmod>
</url>';
}
$sitemapcontent=$sitemapcontent.'
</urlset>';
echo $sitemapcontent;