In order to allow multi-select with php you have to name the form element with brackets. So instead of name="event", it needs to be name="event[]".
That will submit the events selected as an array instead of a text value.
Now that you have an array, you can bind it. Our form bindings should allow you to add it to the page comma-separated. To do it manually, you can chage:
<?php echo((isset($_POST["event"]))?$_POST["event"]:"") ?>
to:
<?php echo((isset($_POST["event"]))?implode(", ",$_POST["event"]):"") ?>