Are you using any of the one-click installs? Those are listed in 3 categories based on whether they work with 4, 5, or both.
I forget what I installed once (maybe PHP Link Directory?), but the installation tripped up on one PHP version, but ran fine on it–so I installed it with 4, then switched to 5.
You can switch back and forth, so if you run into problems with 5, just switch it back.
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