WordPress Unit Tests using MAMP Pro

Note: Work in progress document.

Note: Change the PHP version number as appropriate to your MAMP Pro install.

Add the following you your PATH, making sure that it is first: /Applications/MAMP/bin/php/php5\.3\.6/bin:/Applications/MAMP/bin/apache2/bin:/Applications/MAMP/bin
mv /Applications/MAMP/bin/php/php5.3.6/conf/pear.conf /Applications/MAMP/bin/php/php5.3.6/conf/pear.conf.bak
source ~/.profile (or where ever you made your PATH changes)
sudo pear update-channels && sudo pear upgrade pear
sudo pear config-set auto_discover 1
sudo pear install pear.phpunit.de/PHPUnit
Make an SVN checkout of http://develop.svn.wordpress.org/
Copy wp-tests-config-sample.php to wp-tests-config.php
Edit wp-tests-config.php and give it a fresh database. Repeat, a database with nothing else in it. The unit tests will drop every table in the database, so don't be stupid and give it access to a database that has anything else in it.
(OPTIONAL) Increase the memory limit in wp-config.php: ini_set( 'memory_limit', '256M' );
Add this to your ~/.profile (or equivalent): export WP_TESTS_DIR=~/svn/wp-dev/trunk/tests/phpunit/ (adjust path as appropriate).

You're set!

Troubleshooting

Run which phpunit — if the answer is /usr/bin/phpunit or similar, then you messed up the PATH. Make sure that the MAMP paths are first. You should get an answer like: /Applications/MAMP/bin/php/php5.3.6/bin/phpunit.