As previously mentioned,

<?
for ($i = a; $i <= z; $i++) {
echo $i
}
?>

will print a -> z, then aa, ba, ca , etc -> yz. However, doing this:

<?
for ($i = a; ; $i++) {
echo $i;
if ($i == "z") {
break;
}
}
?>
will print only a->z. It's simple, but it works.