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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.
So if your scripts are OOP Paradigm then do some further research before making the move.
[edited by: Anyango at 10:21 pm (utc) on Oct. 12, 2008]
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.
<? is no longer valid in PHP5 and should be changed with <?php (or you can change setting in your Apache also)
Thats a little incorrect tomda. Short tags work fine in PHP5. They aren`t enabled by default. Just enable them in your PHP.ini file.