The order is A-Z, a-z.

<?php

$init = ord('A');
$end  = ord('z');

printf("Init.: %d | End.: %d\n", $init, $end);

for ($i = $init; $i <= $end; $i++) {
    print chr($i);
}
print "\n";

?>

Apparently it also has non-alpha characters in it, which is why I was confused.

<?php
foreach (range('A', 'z') as $letter) {
  echo $letter;
 }
?>


Yields:
ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz