Código PHP:
<?php
$sel_foo[$_POST['foo']] = ' selected="selected"';
?>
<select name="foo">
<option value="candy"<?= $sel_foo['candy'] ?>>Candy</option>
<option value="bar"<?= $sel_foo['bar'] ?>>BAR</option>
<option value="does"<?= $sel_foo['does'] ?>>Does</option>
<option value="nothing"<?= $sel_foo['nothing'] ?>>Nothing</option>
</select>