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PHP is currently on version 6, so if you're running the latest versions of everything, as you should be for the best security, then I doubt you'll have problems. If they're custom scripts then it could be a bit of a headache.
The one to watch out for the the xml functions as these have been rewritten, for the best, so have to change the way xslt is applied to xml. This is quick, I had to do it, just put in a version check for when we upgrade so the script will work on either PHP4 or PHP5.
As always it is best to read all the documentation about the upgrade on the php website, and use something like agent ransack to check your scripts for any references to functions that may have changed or been deprecated.