And the winner is .... David !
The warning I get is of type "Cannot modify header information - headers already sent by (output started at C:\UwAmp\www\.....\page.php:XX) in C:\UwAmp\www\.....\page.php.php on line YY...."
At XX level, there is print_r($_SESSION);, while YY corresponds to the line where the header('Location: page.php') is located.
I solved the issue by setting output_buffering to On in the php.ini file.
Thank you very much David!Go to the full post