If you are starting fresh, go ahead and install PHP 5. If you already have machines in production but have a development area handy, download and install the current production version of PHP 5. Because it has only been in production for a short time, it will contain bugs and some potentially unfinished work. If you are intrigued by the new features listed in this chapter, I encourage you to follow the production releases as they evolve, installing and testing with each one. You will then be ready to immediately switch to PHP 5 in a production environment, when you are comfortable with the stability of the release.
The changelog and PHP manual always indicate when a new implementation will cause problems in previous versions, so at least read the changelog thoroughly if not the manual entries for your favorite functions. However, the vast majority of PHP 5 focuses on additional functionality rather than completely replacing existing elements, so you may find that none of your scripts require a rewrite. Requiring a rewrite and rewriting scripts for the sake of utilizing new functionality are completely differentyou may want to rewrite to take advantage of new object-oriented functionality, but you may not have to. If your code is primarily procedural (as with the scripts in this book), there's a better than 95% chance that no rewrites will be necessary.