Probably way too late to be of any use to you, but...
The method IsEmpty() has not been declared as a static function (i.e. it's supposed to be called as part of an object) rather than directly (statically).
Basically, rather than FieldManager::IsEmpty(), the call should be made through an object: $obj->isEmpty()
In most cases it's harmless, but it is bad practice (and depending on the function may cause issues). I would guess that in this case, it may not be harmless - if the method is supposed to check one of the objects properties, then it'll fail to do so as it's not being called as part of an object.