Singleton session problem

probably a quick fix but can't figure out where I'm going wrong. I'm setting up a simple singleton Session class, but I'm getting the following error, so i'm obviously not setting things up correctly:

Am I making an obvious mistake here? Thanks for any help

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

class Session { 

    // Session singleton
    protected static $instance;

    private function __construct()
    {
        //start the session
        session_start();

        Session::$instance = $this; 
    }

    public static function instance()
    {
        if (Session::$instance 开发者_如何学Python=== NULL)
        {
            // Create a new instance
            new Session;
        }

        return Session::$instance;
    }
}

You can't output any data before calling session_start(). Make sure there are no echos or prints or anything that spits out data before you instantiate that class.

Maybe this will help...

http://php.net/manual/en/function.session-start.php

For the error: 

Warning: session_start(): Cannot send session cache limiter - headers already sent ... 

this kind of errors would occur, when the encoding of your script file needs to send some headers just after your script starts to execute, 

this happens mostly with the scripts using normal utf8 encoding. 

To overcome the issue, use utf8(without BOM) encoding provided by notepad++ and most modern editors. Using utf8 encoding and cookie based sessions, will result in headers already sent by error.

The problem with headers already sent errors is that you've sent some body content, html, maybe whitespace, ... This Problem can be removed using two ways.

1. Let the creation of the Session be one of the first things of your script - before any output operation through calling Session::instance() in the beginning.

2. Use output buffering. The first instruction should be ob_start() and the last ob_end_flush().