Android : settHeader ("content type", "") - confusion

I have a function with which I want to POST two variables to the php side, after these two variables match and the server processes the result, I want to return result in JSON. As of now my set header property looks like the following:

httppost.setHeader("Content-type", "application/json");

But while reading on at Wikipedia I found that the content type should be application/x-www-form-urlencoded and to accept JSON it should be Accept: application/json I want more clarity on this, how do I modify my code to achieve my desired result? As of now I am using local host and my POST variables seem to be not delivered on the php side. Following is my complete function:

public void parse(String last, String pwd){
        String lastIndex = last;
        DefaultHttpClient http = new DefaultHttpClient(new BasicHttpParams()); 
        System.out.println("URL is: "+CONNECT_URL); 
        HttpPost httppost = new HttpPost(CONNECT_URL);
        httppost.setHeader("Content-type", "application/json");
        try{

            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
            nameValuePairs.add(new BasicNameValuePair("key", password));
            nameValuePairs.add(new BasicNameValuePair("last_index", lastIndex));

            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));



            System.out.println("Post variables(Key): "+password+"");
            System.out.println("Post variables(last index): "+lastIndex);




            HttpResponse resp = http.execute(httppost); 
            HttpEntity entity = resp.getEntity();
            ins = entity.getContent(); 


            BufferedReader bufread = new BufferedReader(new InputStreamReader(ins, "UTF-8"), 8); 
            StringBuilder sb = new StringBuilder(); 
            String line = null; 

            while((line = bufread.readLine()) != null){
                sb.append(line +"\n"); 
            }
            result = sb.toString(); 
            System.out.println("Result: "+result); 



            //  readAndParseJSON(result);
        }catch (Exception e){
            System.out.println("Error: "+e);
        }finally{
            try{
                if(ins != null){
                    ins.close();
                }
            }catch(Exception smash){
                System.out.println("Squish: "+smash); 
            }
        }
        //  return  result; 

    }