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