$_session does not work well with $_GET ???

[jiten]

I am developing a module, where I catch the inputs through the $_POST like this:

$_session[firstname]=$_post["firstname"];
$_session[lastname]=$_post["lastname"];
.
.
and so on....

it works perfectly : as I go to the next PHP file they carry the $_SESSION values ok ... HOWEVER.. there is a variable that is making me mad !! this is one tranferred using the $_GET statement... why ????

I did it:

$_SESSION[checkin]=$_GET["checkin"];
$_SESSION[checkout]=$_GET["checkout"];

but when I go the new PHP files , they are not being carried... the values get lost !!!!

Why is it happening ?

I will appreciate your help... thi is pretty urgent... thanks in advance,

Jiten