Where's the php.ini file

/etc/php.ini

Or here:

/etc/php/php.ini
/etc/php5/php.ini

Or here:

/usr/bin/php5/bin/php.ini

Anyway, you can always find any file named php.ini in this manner

find / -name php.ini

The simplest yet most powerful usage of the renowned find command. By its help I was able to locate the php.ini on my Ubuntu 9.04 Apache2 and PHP5:

/etc/php5/apache2/php.ini