Passing JavaScript array to PHP through jQuery $.ajax

data: { activitiesArray: activities },

That's it! Now you can access it in PHP:

<?php $myArray = $_REQUEST['activitiesArray']; ?>

You'll want to encode your array as JSON before sending it, or you'll just get some junk on the other end.

Since all you're sending is the array, you can just do:

data: { activities: activities }

which will automatically convert the array for you.

See here for details.