Forse cosi (non testato)

codice:
<? 
$numero_news = 5;
ob_start();
include("news.txt");
$content     = ob_get_contents();
ob_end_clean();
$array       = explode('<endofnews>',$content);
$array       = array_reverse ($array);
$text        = implode('<endofnews>',$array);
$array       = explode('<endofnews>',$text,$numero_news+1);
array_pop($array);
$text        = implode('
',$array);
print $text;
?>