Where are PHP libraries installed?

In my inaugural PHP entry, an answer to a simple question: Where are PHP libraries installed?

Answer: I just installed a PHP library named HTTP_Request using PEAR (the PHP Extension and Application Repository), and it was installed into the following directory:


I don't know this for sure, but it looks like I could have just downloaded the HTTP_Request module and extracted it into this directory without using PEAR, but PEAR handles dependencies for you, which is a big time-saver.