print_r($_SESSION); // netu peremennoj varname
function aaa(){
  $_SESSION['varname']=1;
  print_r($_SESSION); // est` peremennaja varname
  return true;
}

<?
session_name('test');
session_start('test');
print_r($_SESSION);
if(!isset($_SESSION['testvalue'])){
   echo "<BR>No Session variable!<br>";
   $_SESSION['testvalue']='gotcha!';
}
function test(){
   if(!isset($_SESSION['funcvalue'])){
      echo "<BR>No Session variable FROM FUNCTION!<br>";
      $_SESSION['funcvalue']='from function!';
   }
}
test();
?>