By monitoring the outcome of a build you are able to provide continuous quality control throughout the development period of a project. The aim is to reduce the effort required in quality control at the end of development by consistently applying small amounts of effort to quality throughout the development cycle.
Under the continuous integration (CI) model developers should consistently integrate their development efforts into the repository. There should be time delay between committing code changes and the new build - this allows developers to recognize and correct potential problems immediately. Of course measures must be in place to flag errors with the build.
The advantage to developers and project managers to having a stable repository to which commits are made and tested are multiple. I don't need to replicate the Wikipedia list here but suffice to say that I've found although development is slowed slightly by needing to correct bugs (lol!) the overall quality of code is improved. A drawback that is mentioned on Wikipedia and actually made itself very apparent to me immediately is the need for a good test suite. You should expect to either assign a developer to coding unit tests or to allocate time for developers to code these as part of their development cycle.
If you're running Ubuntu installing Jenkins is very easy - a version is included in the repositories and so can be installed with apt-get. There is an excellent resource at that guides you through the installation of Jenkins at rdegges.com that will help you get started. I personally found the Jenkins site itself slightly lacking in documentation aimed at first time users, but there is a large community base of users for support. There is a good tutorial for setting up PHP projects here.
In case you struggle to install PHPUnit you should have a look at this bug comment on Launchpad which will help to solve the known "coverage" bug in Ubuntu installs. The following steps are given (and work) to install phpunit on Ubuntu:sudo apt-get remove phpunit
sudo pear channel-discover pear.phpunit.de
sudo pear channel-discover pear.symfony-
sudo pear channel-discover components.ez.no
sudo pear update-channels
sudo pear upgrade-all
sudo pear install --alldeps phpunit/PHPUnit
Note that I have omitted the last step of the process given on the web which installs phpunit again with apt-get. This breaks the installation because the new version of PHPUnit is incompatible with the CodeCoverage filter and you will get this error: PHP Fatal error: Call to undefined method PHP_CodeCoverage_Filter::getInstance() in /usr/bin/phpunit on line 39
If you follow the steps given above and install phpunit with pear you should be okay :-)