parse variable to xml

I would like to parse my variable "valueStations2" into a xml string. Instead of a static sos:FeatureOfInterestId value (here e.g. "stationname") I would like to have a dynamic parsed from javascript. I tried setting my variable in quotes like "valueStation2" for "stationname" but xml does not except. I would like to know how to add a variable to xml. Any suggestions?

  var valueStation2;

  var xml = "<?xml version='1.0' encoding='UTF-8'?> 
  <sos:GetFeatureOfInterestTime     
     xmlns:sos='http://ift.tt/1xt0Rqo' 
     service='SOS' version='1.0.0' 
     xmlns:ows='http://ift.tt/1r1HZSR'> 
     <sos:FeatureOfInterestId>stationname</sos:FeatureOfInterestId> 
  </sos:GetFeatureOfInterestTime>";

// create handler
var request = OpenLayers.Request.POST({
    url: "http://ift.tt/1r1I2hx",
    data: xml,
    callback: handler
});