PHP header("Location: $url");

Miomir Dancevic

Possible Duplicate:
Headers already sent by PHP

Here i have strange problem to protect my pages , to check is session started, if not redirect to login,

    <?php 
require_once ('includes/config.inc.php'); 

// Start output buffering:
ob_start();

// Initialize a session:
session_start();

// Check for a $page_title value:
if (!isset($page_title)) {
    $page_title = 'User Registration';
}

// If no first_name session variable exists, redirect the user:
if (!isset($_SESSION['first_name'])) {

    $url = BASE_URL . 'index.php'; 
    ob_end_clean(); // Delete the buffer.
    header("Location: $url");
    exit(); // Quit the script.

}
?>

I got this error: on line 8:

session_start() [function.session-start]: Cannot send session cache limiter – headers already sent

Can someone post a good solution to check if the session is started, and if not redirect to login page else stay on page??
Txanks