You would have to start with the value string that comes from the db and turn it into an array like this:
php:
$the_selections = explode(", ", $row_YourRS['the values']);
Once you have an array of values you can add in some code per option in the select list to check if the value of that option is in the list. Here is an example of what that code would look like:
php:
<option <?php echo ((in_array("1", $the_selections))?"selected='selected'":""); ?>value="1">one</option>
What I have listed as 1 in this example should be replaced with your recordset values for these options.
If you use this in_array() function it allows you to make the comparison without looping and making an if statement. I'm including a small two page example with the code so that you have a reference for this.
