Looking for the primary php.ini in Ubuntu to change some default settings across Apache?  No problemo.  Here are three different options for finding it:

• Try running this <?php phpinfo(); ?> through your PHP interpreter.  This will show you where your main php.ini file is.
• Or you can try searching for it in the shell:  find / -name php.ini

• If all else fails, give it the old college try, and see if my setting works for you:  /etc/php5/apache2/php.ini (I’m on Ubuntu 9.10 – karmic).

This should just about do it for you, old chap.