Which problem do we face after switching from PHP 4.0 into PHP 5.3.3?

As we all know yesterday PHP released new version of 5.3.3. My question is following:

What kind of possible problems could we have if we 开发者_StackOverflowupdate PHP 4.0 into up to date 5.3.3.? I mean were their any functions or operators on PHP 4.0 that don't work on PHP 5.3.3. and kind of such a problems?

That's it. Thank you.

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See the official PHP migration doc...