Error message: Warning: Cannot modify header information – headers already sent by ….
It might be cause by the encoding of your php file.
Verify that your php file is encoded in UTF-8 without BOM. In Notepad +:
In UTF-8 under Encoding choose Convert to UTF-8
The BOM, or Byte Order Mark, cause the header to be sent prematurely.
More info on how to find files encoded in UTF8-BOM.
If it doesn’t work, check that you don’t send anything before session_start()