Xdebug is pretty easy to install… but listed below are notes for future reference.

First, installed Xdebug by using PECL:

pecl install xdebug

Here is what I ended up putting into my php.ini file:

zend_extension="/path/to/xdebug.so"
xdebug.profiler_enable = 1
xdebug.profiler_enable_trigger = 1

And in the case of Mac OS X 10.5, at least, that “/path/to/xdebug.so” turns out to be something like:

/opt/local/lib/php/extensions/...

Then, just make sure to restart Apache or whatever you are using.