Issues in passing session data

Hi php users,
    I have some questions regarding the way developers uses php sessions.
For instance, most developers uses session_start() on top of every page. The
problem is that after visiting each page, a new session file is created on
the server. Now i made a web application that uses the start_session
function on top of every page as follows:

<?
      if ($PHPSESSID){
               session_start($PHPSESSID);
               if (session_is_registered('valid_user')){
?>

and it works. But now only one session file is created on the server.

Is this the best way of passing session data??

Hantzley