First, install it.
You can download the latest version:

PEARPHP_UML belongs to the PHP framework PEAR. If you have not done it yet, I recommend that you install the core part of this framework, and then that you use the command tool pear to properly install PHP_UML.
This page explains how to install PEAR.

You could also skip the PEAR install, unzip the last release of PHP_UML to a folder, open UML.php, comment the dynamic include to the exception handler, and finally write an include, from your code, to UML.php, but this method can lead to problems later. Moreover, the command line tool phpuml will not be installed.

Once PHP_UML is properly deployed on your system, there are two ways to use it:

  • through a PHP script, which will rely on the API of PHP_UML, described here.
  • through the command line tool phpuml, by specifying the action to perform, and passing the necessary information (folders to scan, files to ignore, output folder, etc). The available parameters of phpuml are described here.