sessioni

[f4br1z10]

salve.. ho questo errore con le sessioni...

Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at C:\site\index.php:9) in C:\site\index.php on line 94

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\site\index.php:9) in C:\site\index.php on line 94
Loggato

e il codice php è questo..

    if ($_GET["reg"]=="true"){
        require 'connect.php';
        $username = $_POST["username"];
        $password = $_POST["password"];
        $query=mysql_query("SELECT * FROM user WHERE username=\"$username\" AND password=\"$password\"");
        if (!query){
            echo "errore nella select";
        }
        if (mysql_num_rows($query)>=1){
            session_start();
            $_SESSION['login']=true;
        }
        else
            echo "non esiste";
            
    } 

dove sbaglio?