In order to run test that uses phpunit-selenium you need to download the
Selenium Server from http://seleniumhq.org/download/ (the actual supported
version of the server is 2.23) and start it with:

  java -jar selenium-server-standalone-X.XX.X.jar

The Selenium Server receives Selenium commands from your tests and
interprets them, and reports back the results of running those tests.

An example phpunit test:
    <?php
    require_once 'PHPUnit/Extensions/SeleniumTestCase.php';

    class Example extends PHPUnit_Extensions_SeleniumTestCase
    {
      function setUp()
      {
        $this->setBrowser("*firefox");
        $this->setBrowserUrl("http://www.debian.org/");
      }

      function testMyTestCase()
      {
        $this->open("/");
        $this->assertTrue($this->isTextPresent("debian"));
      }
    }
    ?>

For this test to run, you have to install iceweasel and make a link
from /usr/bin/firefox to /usr/bin/iceweasel (You may want to save a
copy of the script originally installed in /usr/bin/firefox)

You can find documentation about how to write tests at
http://seleniumhq.org/docs/05_selenium_rc.html