Insert Link (html Content) In all Pages

Hi!
I'd like to know, if any of you would like to explain, how can I insert a text link on bottom of all the pages of one site, or at least html and php.
The reason is because I want to provide free webhosting for my clients (I build sites) but, well, since nothing is 100% free, I want my link to appear at the bottom of their pages.

Honestly I only hope this can be done using .htaccess. I'm not very good in this domain.
Thank you, anyway, for reading!