Undefined offset

<?php
$days = ['first DW'=>'Monday', 'second DW'=>'Tuesday', 'third DW'=>'Wednesday','fourth DW'=>'Thursday','fifth DW'=>'Friday','sixth DW'=>'Saturday','seventh DW'=>'Sunday'];
foreach($days as $key => $d){
    if($d === $days[count($days)-1]){
        echo "$key -> $d";
    } else {
        echo "$key -> $d, ";
    }
}