session_id()

I have myphp.php page and phpBB which is open source.

what i am trying to do is,

when i login into myphp page, and when i click Discussion Board, it can log into Discussion Board page without typing login and password.

So, i am trying to get session_id from phpbb_sessions table.

But, when i click discussion board, it generates another session_id.

i do not know how to connect without typing username and password.

would you please give me some comments???

-----myphp.php---
<?
 
if (isset($_SESSION['username']))
{
 include ("../connect/config.php");
 
 $query  = "SELECT * FROM phpbb_sessions";
 
   $result = mysql_query($query, $link);
   
          if($row = mysql_fetch_array($result, MYSQL_ASSOC))
       {
            
          $query1  = "SELECT sess.*, users.* FROM phpbb_sessions as sess, phpbb_users as users where sess.session_user_id=users.user_id";
          $result1 = mysql_query($query1, $link) or die (mysql_error());
              echo 'There were this many rows: ' . mysql_num_rows($result1);
             
              if($row1 = mysql_fetch_array($result1, MYSQL_ASSOC))
               {
                  echo '<tr>
            <th width="100%" align="left" valign="middle" bordercolor="#CCCCCC" bgcolor="#FFFFFF" scope="col"><a href="../phpBB2/index.php?sid='.$row1['session_id'].'" class="leftnaviDB" target="_parent">DISCUSSION BOARD</a>    </th>
            </tr>';
              }
     }
 }
?>
------
 if i change target="_parent" to target="_self", will it have only one session_id??

i understand when i open a browser, it has a session_id of that opened browser.