Having problems with user defined sessions using postgresql as my session manager

 I have written user defined session management library in php using postgresql as my session repository.  When I let my session timeout on my browser I get the below errors on my screen.

    Warning: pg_query(): Query failed: ERROR: duplicate key violates unique constraint "li_session_pkey" in /var/www/html/liberty/includes/session.php on line 31

Fatal error: Call to undefined function: pg_error() in /var/www/html/liberty/includes/session.php on line 31

Warning: Unknown(): A session is active. You cannot change the session module's ini settings at this time. in Unknown on line 0

 The problem is in the write function of my library. Here it is:

 function pg_session_write($sid, $val) {
     $life = get_cfg_var("session.gc_maxlifetime");
     $exp = time() + $life;
     $selQ = "SELECT sidid FROM li_session
     WHERE sidid = '$sid' AND exptime >". time();
     $selR= pg_query($selQ)or die("It didn't work:". pg_error());
      if(pg_num_rows($selR)) {
         $sesupQ ="UPDATE li_session SET
         exptime = '$exp', vidval = '$val'
         WHERE
         sidid = '$sid' and exptime >". time();
         $sesupR=pg_query($sesupQ) or die("It didn't work:". pg_error());
     } else {
      $sesinQ = "INSERT INTO li_session
     VALUES('$sid', '$val', '$exp')";
     $sesinR= pg_query($sesinQ) or die("It didn't work:". pg_error());    
     }
 }