If you look at the logic in the code, it is like this. It starts the session, checks to see if the variable $_SESSION['count'] has a value. If it does, it changes the variable ($_SESSION['count']) to equal zero, if it doesn't, the variable $_SESSION['count'] will increment (meaning if it was 0, it will be 1, if it was 1, it will be 2, etc.). Then it writes to the page the variable's value, and a link to "stest.php." Now then, you'll realize that you have no <br> tag in there, so the variable will be written right along side of the link. Also, in this function, unless the session variable "$count" has a value, it will become zero. Therefore, try putting ?count=1 at the end of your page and re-running the script. Then test the results.
I've never used sessions before, but this is just what it looks like to me..