So, I just stumbled across this one in another question:
Parent constructors are not called implicitly if the child class defines a constructor.
What else should one be aware of when writing code in PHP from a C# background?
Here is an article that discusses C# from the PHP point of view. This should help you out some with knowing what's alike and what'd different between the two languages.
The differences are too numerous to count, really. I suggest getting some kind of introductory literature on PHP. Both are extremely different languages.