How to implode multiple select?

[picocode]

Hi,

I have an html form with a drop down menu (all days of the week) using select with multiple set to true. What I understand (or not :-) is that a multiple select value is stored in an array when using POST. I wonder how I could implode this array to one single string with comma as separator. Let's say "monday,tuesday,wednesday" etc... I know how to use implode but not how to catch the "multiple post array" in a correct way.

Any help are highly appreciated!

Best regards
Simon