您将在每个循环中覆盖XML.考虑以root开头文档，并在foreach循环之外关闭文档:

You are overwriting your XML with each loop. Consider starting document with root and closing document outside the foreach loop:

$writer->openURI('./cache/xmls/posts-'.$news['MONTH'].'-'.$news['YEAR'].'.xml');
$writer->startDocument('1.0','UTF-8'); 
$writer->setIndent(4); 
$writer->startElement('urlset');

foreach ($newsdata as $news){    
  $writer->startElement('url'); 
  $writer->writeElement('loc',$news['title']); 
  $writer->endElement();
}

$writer->endElement(); 
$writer->endDocument(); 
$writer->flush();