I did one project in PHP after having had extensive perl experience.

The MAJOR pitfall for me was that, in PHP, unless you specify otherwise, a variable is automatically local to any scope.

This is the complete opposite of perl, where unless you say "my" or "local", you see the global variable.

I'm sure there are a lot of other differences, but that's the one that bit me the hardest, and I'm sure it would bite your PHP folks going the other way, as well, especially if they use "local" instead of "my", since local really isn't the opposite of php's "global"...
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Mike