XML : String could not be parsed as xml PHP

I am getting this kind of data from mantis API. Some times its parsing the data correctly but most of the time i am getting the error "String could not be parsed as xml". I have used following code to parse. Please help me. I am new to xml.

  public function get_cities(){      $xmlString =file_get_contents('http://stagingagentapi.bookbustickets.com/Service.svc/APIGetCities?AgentID=&KeyCode=');      $xml = new SimpleXMLElement($xmlString);       foreach ($xml as $out_namespace)      {         $ns = $out_namespace->getNamespaces(true);        $child = $out_namespace->children($ns['a']);        //$child = array_unique($child1);            return $child;     }     }    

The sample XML response I get from the API is:

     <APIGetCitiesResponse xmlns="http://tempuri.org/">      <APIGetCitiesResult xmlns:a="http://schemas.datacontract.org/2004/07/CRS2011AgentApi" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">      <a:CityPair>      <a:FromCityID>2433</a:FromCityID>      <a:FromCityName>Adivare</a:FromCityName>      <a:ToCityID>311</a:ToCityID>      <a:ToCityName>Chiplun</a:ToCityName>      </a:CityPair>      <a:CityPair>      <a:FromCityID>2433</a:FromCityID>      <a:FromCityName>Adivare</a:FromCityName>      <a:ToCityID>2432</a:ToCityID>      <a:ToCityName>Dhartale</a:ToCityName>      </a:CityPair>      <a:CityPair>      <a:FromCityID>2433</a:FromCityID>      <a:FromCityName>Adivare</a:FromCityName>      <a:ToCityID>2431</a:ToCityID>      <a:ToCityName>Gaonkhadi</a:ToCityName>      </a:CityPair>      <a:CityPair>      <a:FromCityID>2433</a:FromCityID>      <a:FromCityName>Adivare</a:FromCityName>      <a:ToCityID>519</a:ToCityID>      <a:ToCityName>Khed</a:ToCityName>      </a:CityPair>      <a:CityPair>      <a:FromCityID>169</a:FromCityID>      <a:FromCityName>Amravati</a:FromCityName>      <a:ToCityID>15</a:ToCityID>      <a:ToCityName>Nagpur</a:ToCityName>      </a:CityPair>      <a:CityPair>      <a:FromCityID>169</a:FromCityID>      <a:FromCityName>Amravati</a:FromCityName>      <a:ToCityID>212</a:ToCityID>      <a:ToCityName>Pulgaon</a:ToCityName>      </a:CityPair>