handling checked checkboxes PHP

Answer 1

You would need to do some AJAX or a form submit to pass that checkbox data to PHP for processing.

But before you can do that, you would need to include a value attribute on those checkboxes. Currently, $_POST['mark'] (if you used POST to submit the form) would be a 0-based array of the checked checkboxes (and only the checked ones).

I would recommend outputting the user id as the value of checkbox to help out with identifying the marked users.

Then you could do

foreach ($_POST['mark'] as $marked) {
    // $marked would contain the value attribute of the checked checkboxes
    // and you could run a SQL query for each value of $marked.
}