在$ _request中获取值数组

I have this code to get xml in array .

$d_atahipay = $_REQUEST['xml']; $xml = new SimpleXMLElement($d_atahipay);

But i always have this error : Fatal error: Uncaught exception 'Exception' with message 'String could not be parsed as XML'

The content of the request by serialize($_request); are :

a:1:{s:3:"xml";s:778:"<?xml version="1.0" encoding="UTF-8"?>
<mapi><mapiversion>1.0</mapiversion><md5content></md5content><result><operation>capture</operation><status>ok</status><date>2015-10-22</date><time>16:23:56 UTC+0000</time><origAmount>78.00</origAmount><origCurrency></origCurrency><idForMerchant>r</idForMerchant><emailClient></emailClient><idClient></idClient><merchantDatas>  <_aKey_idclient></_aKey_idclient></merchantDatas><cardCountry>FR</cardCountry><ipCountry>MG</ipCountry><transid></transid><is3ds>No</is3ds><paymentMethod>VISA</paymentMethod><customerCountry>MG</customerCountry><returnCode/><returnDescriptionShort/><returnDescriptionLong/></result></mapi>
";}

So someone can help me to get the xml content ?

Thanks