I have a next piece of the template:
...';
echo str_replace('value="' . $result['interest'] . '"','value="' . $result['interest'] . '" selected',$list);
?> This involves making a string containing your list, then using the string replace function to find the correct option and adding selected to the tag. If using XHTML you will need to use selected="selected".
http://sandbox.onlinephpfunctions.com/code/37eb8f5a213fe5a252cd4da6712f3db0c5558ae3