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