Microsoft, as one of the largest software developers in the world, will know that, the more popular the software, the more hackers look to find vulnerabilities. PHP is no exception, with its rising popularity amongst even the most die hard .net developers, there are more and more security holes being discovered everyday.
Fortunately, as PHP is an open source project, resolution time for bugs and vulnerabilities are exceptionally fast, and any additional issues that are found are fixed quickly during the beta phase.
Although there is a sizeable lead time between new releases of PHP and web hosts upgrading their own version, owners of dedicated servers can have the upgrade almost immediately. The usual issues that plague an upgrade of PHP are almost always related to compatibility, as more often than not, a small change can break almost any application and it is important to note the changes that occur from version to version.
The best way to guard against possible incompatibilities is to test your scripts against the new version of PHP in a localized environment such as your PC. Thanks to tools like XAMPP, a full Apache PHP mySQL stack can be run on your desktop without installing a single one of the applications.
Download XAMPP and test every single one of your scripts and/or applications against very specific test cases that should cover all facets of its operation. Be sure to keep these test cases and the results documented for future upgrades that will need testing.