You can download PHP 5 from http://www.php.net/downloads.php. The installation process is identical to PHP 4; however, PHP 5 does have some new configuration options. See Table C-1 in Appendix C for a complete list.
During the transition from PHP 4 to PHP 5, it's common to want to run both versions of PHP simultaneously. This lets you easily test code under PHP 5 without switching completely away from PHP 4. It also allows you to slowly migrate scripts from PHP 4 to PHP 5, as you can control which version of PHP parses specific sets of pages.
One option is to run two separate web servers, either on different machines or on different ports on the same machine. The primary advantage of using two web servers is that you can run both PHP 4 and PHP 5 as a module. The disadvantages are that you either need multiple computers at your disposal or need to be familiar with installing Apache (or your particular web server of choice). It's also a lot of work for a temporary situation.
Another solution is to continue using your existing web server and install one version of PHP as a module and another as a CGI. This gets you up and running with both versions with minimal fuss; however, the CGI version of PHP lacks certain features included in the Apache module.
Appendix C provides detailed instructions for setting up PHP 4 and PHP 5 under Apache on both Unix and Windows. It shows how to configure Apache to switch between the versions both on a directory-by-directory basis and by setting up a virtual server on another port.