header() error not shown in php

Question

I wrote a PHP program, I use session_start() and header() functions, I know I should use this functions before I sending anything to client. it\'s ok,

Answer 1

You don't see the error because
a. output buffering is on
b. the server ignores your error_reporting function because something else instructs it otherwise run phpinfo(); and see what it says there about output buffering and about error_reporting.