If you know it's going to come in in that format, you could just do:
$_GET['city'] = explode(",", $_GET['city']);
The only problem you will run into is when a value actually contains a comma.
If you need to get your form to actually submit it that way, it's a different problem. I'll assume you are using jQuery for simplicities sake, but you could do:
$('input[type="submit"]').on('click', function() {
var cities = '';
$('[name="city"]').each(function() {
cities = cities + $(this).val() + ',';
});
cities = cities.substr(0,cities.length-1);
$('[name="city"]').remove();
$('<input name="city" value="' + cities + '">').appendTo('form');
});
This will remove all of your city fields when they click submit, and add a new city field with the values in a comma seperated list. You may need to attach it to the form submit action instead, but I'm not sure how jquery handles elements being added on the submit event.
