where to set session.bug_compat_warn ?

hi, I have the warning message below. In which conf file can I set session.bug_compat_42 or session.bug_compat_warn to off? And where can I find this conf file? (annexe question: Is it possible to get the warnings in a variable so I could print it in html comment, or write it in a log file?) thanks -- Warning: Unknown(): Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively.