Elgg/Elgg — comment on commit 6d0991b93c251473bc37a06bd383c362f9d396a9
File: engine/lib/metadata.php line 311
By brettp:

Invalid means the metastring doesn't exist or the metadata doesn't exist on that entity. IIRC the argument for null was that false could be confusing about the state of the metadata as still being there when it really was never there at all. I don't think this is overly useful and prefer a flat bool return.