请教如何在php里修改xml文件

if ( file_exists ( 'out.xml' )) {
     $xml  = simplexml_load_file ( 'out.xml' );
 
     foreach ($xml->pic->list as $key => $value) {
      	$attr = (array) $value->attributes ();
      	var_dump($attr[[email protected]']['smallinfo']);
      } 
} else {
    exit( 'Failed to open test.xml.' );
}

$s =<<< XML



.html/article/hardware/008.html
http://www.niutuku.com/
http://www.niutuku.com/
http://www.niutuku.com/

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XML;
$xml = simplexml_load_string($s);
for($i=0; $ipic->list); $i++) {
  $xml->pic->list[$i] = $i;
  $t = $xml->pic->list[$i]->attributes();
  $t['smallpath'] = $i;
}
echo $xml->asXML();

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