As you've learned in previous chapters, the symfony framework is a set of files written in PHP. A symfony project uses these files, so installing symfony means getting these files and making them available for the project.
Symfony requires at least PHP 5.1. Make sure you have it installed by opening a command line and typing this command:
> php -v PHP 5.2.5 (cli) (built: Nov 20 2007 16:55:40) Copyright (c) 1997-2007 The PHP Group Zend Engine v2.2.0, Copyright (c) 1998-2007 Zend Technologies
If the version number is 5.1 or higher, then you're ready for the installation, as described in this chapter.