Unfortunately, it looks like MSIE 6, and possibly others, do not reliably set the HTTP_ACCEPT header to reflect the content type being requested. Often MSIE will send a */*.
So at the present moment it's not clear to me how one can recognize a request on the server which resulted from an img src="" line.
It looks like this is a known problem but no good solutions are provided for cases of user contributed content:http://brian.pontarelli.com/2006/05/02/is-your-browser-requesting-a-page-twice
Unfortunately this does not work for MSIE:
Let's say you have a simple PHP script on a page such as:
if ( @$_SESSION["counter"] == NULL )
$_SESSION[ "counter" ] = 1;
$_SESSION["counter"] += 1;
print ("<h1>Counter is currently " . $_SESSION["counter"] . "</h1>" );
You would expect that counter would increment by 1 on each subsequent page load.
HOWEVER, a case can arise where you notice the session counter increasing by more than one on each page load under FireFox but not under MSIE. This occurs, as was the case in some code I was working on today, when you have an empty <img src=""> in your document somewhere.
In this case, FireFox will attempt to pull whatever the current page is as the source for the image, which will load your PHP script page and increment the counter "in the background".
MSIE will attempt to load the default directory index of the current directory as opposed to attempting to load the current page which is why this issue is typically not visible under MSIE.
In the static content case this is not much of a problem as you just have to make sure all of your references back to the server are correct. However, in the case of dynamically generated content and content provided by endusers you are much more likely to run into this situation.
To guard against this case you can check the HTTP_ACCEPT request header before manipulating your session. This works in Firefox but not MSIE, unfortunately. I have not come up with nor found a good cross browser solution.
if (( @$_SERVER[ "HTTP_ACCEPT" ] != NULL ) &&
( preg_match( "/^.*image.*$/", $_SERVER[ "HTTP_ACCEPT" ] )))