Timeword - compress an array of days and represent it as a set of ranges

Question

I am trying to make a function that will take this input: mon,tue,wed,thu,fri,sat

and spit out this: mon-sat

At the moment, i'm using this.

function dayrange($days){
   $days = explode(",", str_replace(" ","",$days));
   return reset($days) . "-" . end($days);
}

Okay now for the curve ball, I want the function to also work if I give it this string: mon,tue,wed,fri,sat

It needs to skip thursday and say something like: mon-wed, fri-sat

Can I get recommendations on the best way to do that? I'm using PHP, so answers in php are great, but psuedo code is fine too.

Cheers