Website Credit PHP Snippet

Hey all
I am wondering if this is even possible:
I would love to be able to add a snippet of code to the footer of my clients’ websites that i build. It would plugin “Designed by ___”.

If I ever wanted to change what it says, or where it links to, I’d have to login to all of them again to change it.

Is there a way to insert a php snippet that would display what I want, and I could change that info somewhere on my end?

I think I saw some premium themes that had this somewhere… that had none of the designed by text it in, but when I deleted the php line, it disappeared.

Any ideas?