Header already sent error after creating custom library (MY_Cart.php) in library folder

[eluser]tkaw220[/eluser]
Hi,

I am getting this error message: Cannot modify header information - headers already sent by ... after I had put a custom library inside my application/library folder. When I tested it in localhost environment, it works perfectly. But it breaks in live server. Any idea how to fix it?

Thank you.

Best regards,
Edwin