Where can I find php.ini?

Today I needed to install the IBM DB2 library. I went through all the steps up to `make install`, and I found ibm_db2.so in `$PHP_HOME/lib/extensions/somecomplicatedname/ibm_db2.so`. The great catch is the last step is to configure file *php.ini*, but there aren't ***any*** *php.ini* files on my system. PHP works fine, except of course for this newfangled ibm_db2 thingamajig. (I tried a small PHP script which fails and indicates that the ibm_db2 functions are not available.) How would I go about finding or setting up the php.ini file so that I can enable the library I need?