Rookie PHP question
Many ways to do this... the first one I can think of is instead of using echo, concatenate all the results into a string, then remove the last , character.
Another way would be to use a for loop instead of foreach and then iterate to the size of $mykey_values - 1 and then print the last one without a ,. And I'm sure others will post other ways (maybe with real code too - my PHP is too rusty for me to risk a real code sample).