Converting amazon MWS scratchpad queries to API calls

I want to know if there is a way to convert my Amazon MWS scratchpad queries to an API call

e.g. when using the MWS scratchpad I'm given a String to sign

   "mws.amazonservices.co.uk"
   ."/Products/2011-10-01"
   ."AWSAccessKeyId=xxx&Action=ListMatchingProducts"
   ."&MarketplaceId=xxx&Query=star%20wars&SellerId=xxx"
   ."&SignatureMethod=HmacSHA256&SignatureVersion=2
   ."&Timestamp=2012-07-27T18%3A59%3A30Z&Version=2011-10-01

After spending days trying to get Amazons order API to work I have given up and have been hoping that the following function would return an xml string...but with no luck

function callAmazon(){
    $apicall =  "mws.amazonservices.co.uk"
   ."/Products/2011-10-01"
   ."AWSAccessKeyId=xxx&Action=ListMatchingProducts"
   ."&MarketplaceId=xxx&Query=star%20wars&SellerId=xxx"
   ."&SignatureMethod=HmacSHA256&SignatureVersion=2
   ."&Timestamp=2012-07-27T18%3A59%3A30Z&Version=2011-10-01   

    $resp = simplexml_load_file($apicall);   //make the call
}

does anyone have any possible suggestions?