This is based on the information on http://developers.sugarcrm.com/wordpress/2008/11/25/enabling-xdebug-under-os-x-leopard/. Download xdebug 2.0.4 (or later - but this is based on the 2.0.4 build) - extract it.

phpize
MACOSX_DEPLOYMENT_TARGET=10.5 CFLAGS="-arch ppc -arch ppc64 -arch i386 -arch x86_64 -g -Os \
     -pipe -no-cpp-precomp" CCFLAGS="-arch ppc -arch ppc64 -arch i386 -arch x86_64 -g -Os -pipe" \
     CXXFLAGS="-arch ppc -arch ppc64 -arch i386 -arch x86_64 -g -Os -pipe" LDFLAGS="-arch ppc \
     -arch ppc64 -arch i386 -arch x86_64 -bind_at_load" ./configure --enable-xdebug
make
sudo make install

My machine didn't have an existing php.ini - so I changed the next section:

sudo cp /etc/php.ini.default /etc/apache2/php.ini

Now - add the following to the end of the new php.ini:

zend_extension=/usr/lib/php/extensions/no-debug-non-zts-20060613/xdebug.so
xdebug.remote_enable=1
xdebug.remote_host=localhost
xdebug.remote_port=9000
xdebug.remote_autostart=1

Finally - tell apache which php.ini to load - add the following to /etc/apache2/other/php5.conf:

PHPIniDir /etc/apache2

Finally - restart apache. For debugging - Eclipse, Komodo or MacGDBp