- substr: documentation ( source)
- stripslashes: documentation ( source)
<?php
$x = '"<?xml version=\"1.0\" encoding=\"UTF-8\"?><checkOrderResponse performedDatetime=\"2017-07-09T16:11:52+03:00\" code=\"0\" invoiceId=\"2000729266842\" shopId=\"45951\"\/>"';
$x = stripslashes($x);
$x = substr($x, 1, -1);
echo $x;
//$data = new SimpleXMLElement($x);
//echo $data->asXML();