Ok, i´ve resolved the issue.
The ´cause of this issue, is that Internet Explorer 6 (versions in English and german) have problems with session., I don´t know if just with php 5, but, it makes a newone,every page you open. so the data in the session get lost.
For avoiding this, you must name your session.
seesion_id("mysession"); //for explample.
and then in every page after starting the session (session_start()), you must make sure the session is not writed it self.
if(!isset("mysession")){
session_start();
}
else{
//redirect to the page where the session is created
}
At least, this worked for me, and there are several cases like mine.