Save a incoming url parameter as a session variable

I want a user to be able to filter out questions written in a specific language, and do it using an url-parameter when entering the site. 

I am in qa-base.php accessing the url-parameter, but I can't seem to keep the variable value when I switch to some of the other pages. Trying with using session variable, but cant get it to work. 

    $thelang=qa_get('thelang'); // get from URL parameter if possible

    if(isset($thelang)){

        $_SESSION'thelang'] = $thelang;

        echo "<br>Got GET Parameter!<br>";        

    } else { // get from SESSION variable

        $thelang = $_SESSION'thelang']; // this part doesn't seem to work.

    }

Thanks for any help! Maybe there is better ways to save the variable between pages.