Pre-selection can be accomplished on HTML form controls the following ways (attributes):
checkbox: checked="checked"
radio button: selected="selected"
select (drop list): selected="selected"
You simply need to add code to check for the condition and add this attribute if the previous value was selected:
Assume $choice=2:
echo '<select name="dropbox">;
echo '<option value="1" ';
if($choice==1){echo' selected="selected";}
echo '>Choice1 Text</option>';
echo '<option value="2" ';
if($choice==2){echo' selected="selected";}
echo '>Choice2 Text</option>';
echo '<option value="3" ';
if($choice==3){echo' selected="selected";}
echo '>Choice3 Text</option>';
echo '</select>';
This code will have Choice2 Text preselected. I frequently add an asterisk (*) to the end of the selected choice text (using if statements) to indicated this was the current value on entry into the page.
If your option choices come from a database the code actually gets a bit cleaner...