I wanted to use memcached but didn't want to compile all the dependencies by hand and wanted to use the stock version of Apache that ships with OS X so I cobbled together the following instructions.

1. Install MacPorts
2. Use MacPorts to install memcached:
   
   sudo port install memcached
3. Use pecl to download the latest version of memcached and uncompress it:
   
   cd /tmp
pecl download memcached
tar xvzf memcached-*.tgz
4. Build the extension using MacPort's version of libmemcached (which was installed as a dependency of memcached):
   
   cd memcached-*/
phpize
./configure --with-libmemcached-dir=/opt/local/
sudo make install
5. Enable it by editing php.ini and adding the following line:
   
   extension=memcached.so
6. Restart Apache:
   
   sudo apachectl graceful

References:

• Install memcached PHP Extension on OS X Snow Leopard