Select a random file from folder, distribute probability based on name

The files are fonts, it's common for them to have names like FontName-type.extension

I have them all together in a single folder, I currently have this function:

function random_font($dir = 'fonts')
{
   $fonts = glob($dir . '/*');
   return $fonts[devurandom_rand(0, count($fonts) - 1)];
}

The goal is however to get as representative a sample as possible, and fonts that share a FontName are much more similiar to each other than those that don't.

My problem is selecting a font at random such that fonts that share FontName also share the probability of being selected.