JavaScript data post

I am working in PHP I have created a page here I have 2 combo box when I select first combo box item the second one is filled according the first combobox using JavaScript the problem I am facing is where I am trying to save the data of second combo box in database I found null value. transition_party_id is the combobox that fill dynamically here I code

function getXMLHTTP() { //function to return the xml http object
    var xmlhttp=false;  
    try{
        xmlhttp=new XMLHttpRequest();
    }
    catch(e)    {       
        try{            
            xmlhttp= new ActiveXObject("Microsoft.XMLHTTP");
        }
        catch(e){
            try{
            xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
            }
            catch(e1){
                xmlhttp=false;
            }
        }
    }

    return xmlhttp;
           }

    function getparty(category) {       
    var strURL="../admin/get_partyname.php?category="+category;
    var req = getXMLHTTP();

    if (req) {

        req.onreadystatechange = function() {
            if (req.readyState == 4) {
        开发者_如何学运维        // only if "OK"
                if (req.status == 200) {                        
                                                   document.getElementById('partydiv').innerHTML=req.responseText;                      

                } else {
                    alert("There was a problem while using XMLHTTP:\n" + req.statusText);
                }
            }               
        }           
        req.open("GET", strURL, true);
        req.send(null);
    }

}

req.open("GET", strURL, true);
req.send(null);

You send 'null' post data by "GET" method.

Or you waiting data in get?