Maybe the title is a bit unrelated to the post as I can’t seem to find a valid and justified con against it.
Anyway, some thing you should know: PHP 5 was released cos it contains dozens of fixes, upgrades. It wasn’t released cos the team which manages the project had a godly touch or they were drunk and when they woke up all they knew was they have to release a new PHP. No. There was a very good reason, and just for the matter of the argument, when something is free anyway, why wouldn’t you upgrade to a better, improved version? Huh?

The guy who I work with day-by-day, and probably my best friend as well, will say “don’t fix what is not broken”. I say, if something contains bugs, isn’t that thing broken?

So, as I said, you have nothing to fear of, much likely your code will work on the new platform as well, sincerely, when I upgraded to PHP 5 none of my scripts stopped working and there are a few hundred of separate scripts, but just to be sure, let’s check some things.

First of all, i don’t know if you know or not, there are some reserved words you can’t define in your scripts as variables or constants. That’s clear I guess, the “why don’t” has a simple answer… well, two: the first is, cos this is what the developers of PHP said, secondly cos why would you want to define a constant which was previously defined in the PHP core as a function. For example, “echo”, would you redefine it as a constant? I hope you wouldn’t. Also, the same applies to reserved words like “FALSE”, “TRUE”, “NULL”, etc. as these are reserved constants. Have a look on php.net on the reserved keywords list, well, table (link). Also, someone asked me of why “parent” and “self” throws errors when using them in scripts as (re)defined constants. Well, they are inbuilt classes, simple.
Another major change is that array_merge() accepts only arrays. This should be obvious anyway but as of PHP5 it returns NULL. There are some other changes as well, php.net has a more comprehensive list here (link).

And probably that was all you feared of. Really.