What type of encoding is this string?

I have an encoded string: VH+_+smN+IPimspbC+_+ygX73g==

The original string is: 51756

So 51756 => VH+_+smN+IPimspbC+_+ygX73g==

What is the encoding algorithm type?

How can decode VH+_+smN+IPimspbC+_+ygX73g== to original string?