In the latest post to her site Lorna Mitchell has posted a helpful hint on how you can run multiple versions of PHP at once, mostly how to get the latest version without messing up your current install.
When I advise people about upgrading their PHP version, I say things like "just run your test suite with the new version" "just grab the new version and try your site with the built-in webserver". A couple of people recently have asked for more detail on how to actually achieve these things so here's a quick primer on getting new PHP without touching anything to do with your existing PHP installation.
You'll need a bit of knowledge around compiling software to get the job done, so if you're only used to aptget-ing or yum-ing you might be a little lost. She does include all the commands you'll need including the special "prefix" flag on configure telling it to put PHP in a different location than normal. She also includes a brief test to ensure that it's all up and working (using the built-in web server).