The default compile of php no longer installs a php.ini file.
There are two included with the php distribution php.ini.dist and php.ini.recommended(?).
The place to put the config file is in /etc/php.ini (if using Apple's directives) or /usr/local/etc/php.ini (if using the default configure, or Mark's compile). The quick way to find out (assuming php4.3+) is
bash-2.05a$ php -r "phpinfo();" | grep Configuration
Configuration File (php.ini) Path => /etc
If you can't do that (because you are using pre 4.3, for shame!) then try the <?php phpinfo() ?> script another post mentioned.
Hope this helps,
tychay at php dot net