Try this in a script:
<?php
  exec( "/bin/sleep 30 >/dev/null 2>&1 </dev/null &" ) ;
  echo "Done !!" ;
?>

Now call it from the browser several time (Reload button). If you type a ps ax command you will see several "sleep 30" processes. Wait all the processes to stop.

Now put session_start() as the first line. Press RELOAD: it completes OK. Now press RELOAD again: it hangs !!! It waits for the sleep to terminate before starting the next !! So if you press RELOAD 10 times you need to wait 300 seconds !!

Is it a normal behaviour ? How can I have a process completely in the background ? 

Francesco
PS: I tried to put the nohup in the exec string with no success