Laravel 5.3/5.6 collection undocumented method add()?

Both mentioned versions (I have to use 5.3 at work) of Laravel have a Collection method called push() but it seems there is also an undocumented (I checked both documentation pages) method add().

I wonder where the difference is or if add() is an old-lost piece of code? For now I use push() as this is documented and exactly what I need.

via Chebli Mohamed