Unity PHP Post doesn't send POST variable

I use WWWForm, It's not working with ssl connection,

if I use http:// connection on secure domain it doesn't send POST variable and if I use https:// it given error

unknown error

So it just working and send variable only non-secure domain

Unity3d version is 2017.3.0p1

I tried windows and mac version

Here is c# code:

    WWWForm form = new WWWForm();


    form.AddField("a", "var1");
    form.AddField("b", "var2");
    string url = "http://www.example.com/ajax/test.php";

    WWW w = new WWW(url, form);
    yield return w;
    if (!string.IsNullOrEmpty(w.error)) {
        Debug.Log(w.error);
    }
    else {
        Debug.Log(w.text);
   }

It returns only test.

Here is test.php :

<?
echo 'test '.$_POST["a"];
?>

Please assist on me, what should I do?