How to use a session variable globally

Hi,

i want to use an incoming id, for example:
https://www.wordpressdomain.com/category/title.html?id=123

I safe it in the index.php to a session variable, for example:


<?php session_start()?>
<?php $_SESSION["sub_id"] = $_GET["id"];?>


On every other php file of my template, i include

<?php session_start()?>


Then i want to pass the id over to an external link like this:

https://www.externaldomain.com/10362533?SID=<?=$_SESSION["sub_id"]?>


HERE COMES MY PROBLEM:

Everything is working, UNLESS the user is landing on my blog AND surfing around before she clicks on my link for the external domain!

So my question is:
Where to implement the session-stuff (which files) so that wordpress handles my session all the time the user stays on my page?
I seem to lose the variable somewhere on the way when the user surfs my site…

Any ideas?

Thanks,
Hans