session_start() creates a session (or resumes the current one based on the session id being passed via a GET variable or a cookie).
If you want to use a named session, you must call session_name() before calling session_start().
This function always returns TRUE.
Note: If you are using cookie-based sessions, you must call session_start() before anything is output to the browser.
session_start() will register internal output handler for URL rewriting when trans-sid is enabled. If a user uses ob_gzhandler or like with ob_start(), the order of output handler is important for proper output. For example, user must register ob_gzhandler before session start.
Note: Use of zlib.output_compression is recommended rather than ob_gzhandler