It does not have one by default.
I'm on Tiger, and I custom installed PHP 5.1, but I left the default PHP 4 in place.
See if you have the file /etc/php.ini.default or /private/etc/php.ini.default. If so, rename that to php.ini, and there is your config file for the default PHP install on OS X.
The other option is to not use the default PHP install. Marc Liyanage has some great PHP packages for OS X at http://www.entropy.ch/software/macosx/php/
@$moke, use one of Marc's PHP packages, and you can get install packages for MySQL for OS X from mysql.com