Anytime you use session_start();, you have to be sure it is called before any output is sent to the browser. (echo, print) or any white space will cause PHP to flush it's self, which will cause an error if your trying to use session_start(); after the flush.
You have (2) options...
1. make sure your not sending any output before the session call
OR
2. Use the
ob_ functions to hold your output until you call the session_start();
me!