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How to capture unset REQUEST values in PHP
[CONTENT_TYPE] => application/x-www-form-urlencoded
 [QUERY_STRING] =>

$pos=strpos($_SERVER["QUERY_STRING"], 'xml');
$xml="";
if($pos!==false){
    $xml = substr($_SERVER["QUERY_STRING"], $pos+strlen("xml="));
}

if (count($_GET)>1 or is_not_well_formed_xml($_GET['xml'])) {
   $xml = substr($_SERVER['QUERY_STRING'], 4);
   if (is_not_well_formed_xml($xml)) {
       really_fail();
   }
}

<root><link href="http://example.org/?querystring#fragment"/></root>