Some of us still use (and like) PHP, thank you very much. Nothing wrong with targeting a niche.

If it actually required PHP, or was somehow particularly useful to PHP users, or was even just identified on the website as a "PHP/CSS framework", that'd make sense. This isn't targeting a niche, it's firing a rifle blindfolded and hitting your neighbor's niche.

I couldn't find on the website how exactly they use PHP. Does a website need to be coded in PHP to use Foundation?

No, it doesn't, which is probably why nothing on the site even mentions PHP. Nonetheless, when you download Foundation it's presented PHP files that do just enough in the way of conditional server-side includes to scare away people that don't know PHP and don't want to have to unravel it out to try out a new CSS framework. That's why it's frustrating.