Not OP, but the only one that I know of and that can trip you up without giving any form of warning is the change of resources types to class types for some of

Not OP, but the only one that I know of and that can trip you up without giving any form of warning is the change of resources types to class types for some of the libraries, like curl. Thus if your old code did a check like is_resource it will now return false instead of true.