Link to .php page with session

Hi, I have a .php page that use sessions and shows a file .FLV by transcoding, on the fly, from any format video to FLV using a shell command [ system("/usr/bin/ffmpeg -re -i ./file.wmv -f flv -an -sameq -") ]. In a .php page: case 1): if j put a link to any page without session, all is ok; case 2): if j put a link to .php page with session -->

        when j click the link, the link take place only after
        system() command execution end.

Is there a way to obtain case 1) behaviour also in case 2) ? Thanks Sante Luciani