how to create soap xml request in php

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY" xmlns:soap="http://ift.tt/sVJIaE">
  <soap:Body>
    <SubmitRequest xmlns="http://ift.tt/1jXFbCI">
      <siteID>string</siteID>
      <username>string</username>
      <password>string</password>
      <xmlFormattedString>string</xmlFormattedString>
    </SubmitRequest>
  </soap:Body>
</soap:Envelope>

we have to call above soap xml. i have siteID, username, password. where string is below

<ArnRequest><Availability DisplayCurrency="USD" SearchTimeout="15"><HotelAvailability InDate="2007-04-26" OutDate="2007-04-29" Rooms="1" Adults="2" Children="0"><Hotel HotelID="8800"/></HotelAvailability></Availability></ArnRequest>

i have no idea on soap request. Please help with this to get response on above soap xml in PHP. The above xml is of ARN(Alliance reservations) thanks in advance.