The changes to empty() also affect isset() in a similar way.
$foo=""; isset($foo) returned false..
but under php4 it returns true, because the variable is defined.
I discovered this when converting other people's code to php4. They used isset() unnecessarily - with the implicit expectation it just tested the value of the variable as being non-null. Although I don't agree with sloppy coding, it can give an unexpected shock when run under php4 and so any uses of isset() should be searched for before you attempt to migrate.