first, for a multiple select list to send all options, the name of the list needs to have "[]" at the end:
<select name="myList[]" size="3" multiple id="myList">
<option value="1">One</option>
<option value="2">Two</option>
<option value="3">Three</option>
<option value="4">Four</option>
<option value="5">Five</option>
</select>
this will send all of the selections as an array, you need to use the impload() function to convert the array to a comma separated list:
<?php echo((isset($_POST["myList"]))?implode(", ",$_POST["myList"]):"") ?>