I have a file, which starts `<?php session_start();?>`, and it returns an error

```
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at C:\xampp2\htdocs\index.php:1) in C:\xampp2\htdocs\index.php  on line 3

```

and now take a look at the moment, i can't anderstand anyway
when i copy the all content of my file into another file, it start working.
can somebody explain how can it happen.
thanks

update
------

**i havent't any white spaces, at least i can't see them**