Problem login in on admin after i put the files on a real server

Everything is working perfect on localhost but when i put it up on the web i get this error:

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /customers/2/7/a/ in /customers/2/7/a/ on line 3 Login successful! Warning: Cannot modify header information - headers already sent by (output started at /customers/2/7/a/ in /customers/2/7/a/ on line 15

Why is it working locally but on on a webserver?

You must have a higher error reporting level set on your server than on localhost, which is why you’re suddenly seeing the warning errors now.

Remember that the session_start() function will, by default, use cookies to store the unique session identifier in (unless stated otherwise, in which case it will be passed in the URI via the query string). Because cookies are apart of the HTTP header, they must be sent out before any content has been output to the browser. This means that when invoking the session_start() function, it must be done at the start of the script, preferably before any other PHP code (because any error caused by the preceding PHP will be output on the screen, and will therefore trigger the header error you see above).