Is there any difference between these two commands?
<?set "i" = 0 ?>
<?set i = 0 ?>
Quoting variable names is accepted, but is it necessary?
There is no difference. The quotation marks should only be necessary when the thing you are entering contains spaces (since spaces are used as separators). Since a variable name is normally not entered with spaces, the quotation marks generally aren’t needed for variable names that are being set.