如何防止浏览器为javascript和php加载的图像打开新会话

Question

I have designed a system to get some user data(screen resolution and etc) by javascript. In HTML files, by javascript, screen resolutions will be send to server via get parameters of a url(an image). the problem is that when the image is requested from server, a new session instance will be created, while I expect that the session for getting the image would be the same as the session of the parent page. please, if you have any idea how could I resolve this problem. thank alot.

the codes I am using are as follows:

I have put a javascript in all of my web pages to get screen resolution. Here is the javascript code:

 <script language="JavaScript">
 document.getElementById("mypictureid").src="http://www.mywebsite.com/getresolutionpic.php?w="+screen.width+"&h="+screen.height;
</script>

and the html of the pages includes:

<img id="mypictureid" src="http://mywebsite.com/otherimage.gif"

and the following php code will be executed with each page loading to store user data in $_SESSION.

$some_name = session_name("generalvisit");
session_start();
$_SESSION['entertime']=new DateTime();

and to track the user a userid is created by a mysql database and stores in $_SESSION['userid']

if(!is_numeric($_SESSION['userid'])) $_SESSION['userid']=getNewUserID();

now, when the getresolutionpic.php is triggered by javascript, store screen resolution of that user stores in the $_SESSION variable of the same user. In getresolutionpic.php we have:

$some_name = session_name("generalvisit");
session_start();
if(!is_numeric($_SESSION['userid']))
{
    $_SESSION['userid']=getNewUserID();
    $_SESSION['entertime']=new DateTime();
}
$_SESSION['width']=$_GET['w'];
$_SESSION['height']=$_GET['h'];

The problem is that when getresolutionpic.php is called, $_SESSION['userid'] is empty and a new userid will be created while the user is not actually new.

Do you have any idea where it goes wrong?