$xml = new SimpleXMLElement(file_get_contents($file));
$xml->registerXPathNamespace("def", "http://schemas.datacontract.org/2004/07/Pull.BLL.Models.OTA");


for this xml? (just copying a small part of the xml to be able to see the namespace)

<?xml version="1.0"?>
<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"><s:Body><GetHotelAvailResponse xmlns="http://tempuri.org/"><GetHotelAvailResult xmlns:a="http://schemas.datacontract.org/2004/07/Pull.BLL.Models.OTA" xmlns:i="http://www.w3.org/2001/XMLSchema-instance"><a:Criteria><a:AvailableOnlyIndicator i:nil="true"/><a:Criterion><a:Address><a:AddressLine i:nil="true"/><a:CityCode>1109342</a:CityCode><a:CityName i:nil="true"/><a:CountryCode>40</a:CountryCode><a:CountryName i:nil="true"/><a:PostalCode i:nil="true"/><a:StateProv i:nil="true"/><a:StateProvCode i:nil="true"/><a:ZoneCode i:nil="true"/><a:ZoneName i:nil="true"/></a:Address>