<?php
function dayrange($days){
    $wdays = array("mon","tue","wed","thu","fri","sat","sun");

    $indays = explode(",", str_replace(" ","",$days)); // expand the list to an array


    $retstr = array_shift($indays); // get the first date

    $curpos = array_search($retstr, $wdays);  // current position in the wdays array
    $intv = 0;    // interval between days to avoid mon-tue like output

    foreach($indays as $d) {
       if($d == $wdays[$curpos]) {
          $curpos = ($curpos++) % 7; // this will take care of wrapping.
          $intv++;
       } else {
           $retstr.= ($intv > 1 ? "-".$d:",".$d); // use appropriate join
           $intv = 0; // reset interval
       }
    }
    if($intv > 0) {   // if anything was left deal with the end.
        $retstr.= ($intv > 1 ? "-".$d:",".$d);
    } else {
        $retstr.= ",".$d;
    }
    return ($retstr);
}
echo dayrange("mon,tue,wed,fri,sat");

mon,tue,wed,fri,sat,sat