How to use POST method to enter data to web service?

Hi, I am newbie to scripting.

I had a web service where i can enter an id and retrieve data from database. So I want to enter access the web service through Unity and enter the id.

When i run the code, the debug log show me this message:WWW.Error:500 Internal Server Error.
But when I remove the form from my www, the error message not showing again.
It seems my AddField part have some problem but i don’t know why.
Can someone point out what is going wrong in my code?

Sorry if my English grammar not good.

Here is my code:

void Start()
    {
        WWWForm form = new WWWForm();
        string url = "http://localhost:54659/WebService.asmx?op=RecipeID";
        form.AddField("id", 1);
        WWW www = new WWW(url, form);
        StartCoroutine(WaitForRequest(www));
    }

    IEnumerator WaitForRequest(WWW www)
    {
        yield return www;

        if (www.error == null)
        {
            Debug.Log("WWW of!:" + www.text);
        }
        else 
        {
            Debug.Log("WWW.Error:" + www.error);
        }
    }

500 Internal Server error means that you have a problem in your php file that handles the forum.

Your php is not compiling correctly with the information you are submitting to it.