This error usually occurs when "session_start ()" is below any data output to the browser, it can be html tags or syntax errors in code snippets that precede the function. Example:
Correct form:
<? Php
Session_start ();
Echo "Site Content!";
?>
Incorrect form:
<? Php
Echo "Website Content!";
Session_start ();
?>
For more details visit:
http://www.php.net/manual/en/function.session-start.php