SOAP send xml string

I have a webservice in PHP, using SOAP and one of sending parameters is 'fileContent' with is a string with xml content like this:

$string='<?xml version="1.0" encoding="UTF-8"?><rootTag><a>tag</a></rootTag>';

The request is

     <!--Optional:-->
     <ws:fileContent>'<?xml version="1.0" encoding="UTF-8"?><rootTag><a>tag</a>  </rootTag>'</ws:fileContent>
  </ws:ReservationDetailsRQ>

The response is

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://ift.tt/sVJIaE">
 <SOAP-ENV:Body>
  <SOAP-ENV:Fault>
     <faultcode>SOAP-ENV:Client</faultcode>
     <faultstring>Bad Request</faultstring>
  </SOAP-ENV:Fault>
 </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Can you tell how can i do this?