www.progress not work with .php url

hi, i'm freaking out with whis problem... I have to take the www.progress from a php file, but seems like impossible take the www.progress from an URL with php extension.

this is my test script:

     IEnumerator OnResponseReceivedTest (string url)
     {
         WWW www = new WWW("http://www.mysite.it/script.php");
 
         while (!www.isDone) {
             Debug.Log ("progress = " + www.progress);
             yield return www.progress;
         }
         
         yield return www;
     }

if my url point into a file jpg,html,txt or anything except php there's no problem, but with php files i obtain just a lot of "0" and "1" when the www request is over. why???

Thanks