How to creating multiple Sitemaps in PHP in this code

i have following problem, i generated the urls for the sitemap after url 50K this code wont make new sitemap like sitemap2.xml or sitemap3.xml

header('Content-type: application/xml; charset="ISO-8859-1"', true);
$dataAll1 = scandir('cache-data');
unset($dataAll1[0]);
unset($dataAll1[1]);
unset($dataAll1[2]);
$sitemap = '<?xml version="1.0" encoding="UTF-8"?>
           <urlset xmlns="'.PROTOCOL.'://www.sitemaps.org/schemas/sitemap/0.9">';

$sitemap .= '<url>
                        <loc>' . SITE_HOST . '</loc>
                        <priority>1.0</priority>
                     </url>';

foreach($dataAll1 as $val){
    $data = json_decode(file_get_contents('cache-data/'.$val),1);
    if($val=='index.php'){
        continue;
    }
    $sitemap .= '<url>
                   <loc>'.SITE_HOST . '/blog/' . $data['jk'].'jktk'.$data['tk'] . '-' . $service->slugify($data['title']).'</loc>
                        <priority>0.9</priority>
                        <changefreq>daily</changefreq>
                     </url>';
}

$sitemap.='</urlset>';
echo $sitemap;

how to write the correct code for the code above so that you can create a new sitemap after reaching 50K urls?

all help is very valuable