How Do I edit a Variable?

[The-Ensemble]

Ok, I'm coding my profile/member info template and I want to use the Last Post thingy.
The one I found is this:

<a href="$userinfo[lastposturl]">$userinfo[lastposttitle]</a>

And it gives the thread title which is what i want to change.
I want to change it so its

<a href="$userinfo[lastposturl]">Last Post</a>

So clicking last post will take you to their last post.

And I need to edit the variable to change it from title to the words Last Post.

How would I do this?

PS: Sorry If i confused you, I've confused myself