I would assume by that error that you're actually trying to enforce a session ID yourself? I have heard of no problems since around PHP 4 of PHP issuing invalid characters. I know there was an issue in PHP of the session ID sometimes being null.
You could do the following, but I wouldn't recommend it because if there's a function you've written that's not performing well, then you'll want to correct it. For example, if the session ID is set as all invalid characters then the following would leave you with an empty session.
already sent (output started at /**/**/index.php:2) in /**/***/index.php on line 2
Are you sure something hasn't already been sent to the document because even a space will cause the message headers to be sent and no session or cookie data can be sent after that. That is what it looks like is happening here. Wh