This should be changed in your php.ini file (your server's PHP configuration), not a file that is Moodle specific. If you are on shared hosting then you should check for hosting documentation on editing php.ini or contact your hosting provider to change this for you.
Read more at a previous discussion: http://moodle.org/mod/forum/discuss.php?d=101761
Read more at a previous discussion: http://moodle.org/mod/forum/discuss.php?d=101761