PHP system that exports the data from 1 table row in a database to a text file.

So basicly, I have a table in my MySQL database, that shows stuff like this: Computer Name: VALUE Key: VALUE.

Now I want an option for people to export this to a text file, but ONLY the Key row.
So for example: On the website it looks like this:

Computer: My-PC | Key: 2250021
Computer: ASUS | Key: 2206693

Now on the text file it should look like this:
2250021
2206693
2869682
6601873

I don't think this is too much work, please respond if you can do this!