change this line:
$_SESSION['PBksID']=$_GET['job_id'];
to:
if(isset($_GET['job_id'])) $_SESSION['PBksID']=$_GET['job_id'];
this way the session will only get set if the job_id querysting variable exists.
My bet is that the session is getting reset at some point to be empty, and that is why it is not carrying over to the other pages.
once it is set, you should be able to use:
echo "Job identification=". $_SESSION['PBksID'];
on any page to display the session value.