Error:

Warning: session_start() [function.session-start]: The session id contains invalid characters, valid characters are only a-z, A-Z and 0-9 in /**/******/index.php on line 2

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /**/**/index.php:2) in /**/***/index.php on line 2

How can we prevent from this kind of error. Do we have to check the content of the session for valid contents only or is there any other way.