A few years ago I installed Apache 2.2x and PHP 5.3.1 on a Linux server I maintain. I used .tar.gz\'s and built them as instructed (instead of rpms and what-have-you). And
On the command line execute:
php --ini
You will get something like:
Configuration File (php.ini) Path: /etc/php5/cli
Loaded Configuration File: /etc/php5/cli/php.ini
Scan for additional .ini files in: /etc/php5/cli/conf.d
Additional .ini files parsed: /etc/php5/cli/conf.d/curl.ini,
/etc/php5/cli/conf.d/pdo.ini,
/etc/php5/cli/conf.d/pdo_sqlite.ini,
/etc/php5/cli/conf.d/sqlite.ini,
/etc/php5/cli/conf.d/sqlite3.ini,
/etc/php5/cli/conf.d/xdebug.ini,
/etc/php5/cli/conf.d/xsl.ini
That's from my local dev-machine. However, the second line is the interesting one. If there is nothing mentioned, have a look at the first one. That is the path, where PHP looks for the php.ini.
You can grep the same information using phpinfo() in a script and call it with a browser. Its mentioned in the first block of the output. php -i does the same for the command line, but its quite uncomfortable.