When someone clicks "back", how do I get the user to the same position? (PHP)

When someone clicks on a link, it sends them to it. But when开发者_运维技巧 a user clicks back, i want it to be at the position where they were before. What is the name for this and how do I go about doing this?

Like on SO, when I click "back", the scroll bar isn't at the top but rather at the position where I was last.

There is no name for it and nothing you can do to make it happen explicitly, but it happens by itself in most browsers.