运行脚本10分钟后出现500内部错误

Question

I am trying to achieve following:

1. Parse XML sitemap of website (working)
2. Download source of every single page
3. Echo something like "Done" or url of website
4. Sleep for few seconds to avoid crashing server

I don't know why, but after about 10 minutes I am always getting 500 ERROR.

<?php
error_reporting( E_ALL );
ini_set('display_errors', 1);


ob_start(); 


$urls = array();  

$DomDocument = new DOMDocument();
$DomDocument->preserveWhiteSpace = false;
$DomDocument->load('http://elody.cz/sitemap.xml');
$DomNodeList = $DomDocument->getElementsByTagName('loc');


//parsovani xml, vkladani linku do pole
foreach($DomNodeList as $url) {
    $urls[] = $url->nodeValue;
}

echo 'Loading XML done!<br />';                  


foreach ($urls as $url) {     
      $data = file_get_contents($url);      
      echo $url."<br />";
      ob_flush();
      flush();
      unset($data);
      unset($var);      
      sleep(1);

} 

ob_end_flush();     
?>

Do you have idea how I could find out where is the problem? :)

Thank you Filip