php: How to list from a to z, aa to zz, aA to aZ, and so on

$length = 2;
$limit = str_repeat('z', $length);
for($i='a';$i<=$limit;$i++) {
    if($i==str_repeat('a', $length+1)) break; // stop on 'aaa';
    myCallback($i);
}

a
b
c
...
z
aa
ab
ac
...
zz

function generate($length = 1) {
    $small = range('a', 'z');
    $capital = range('A', 'Z');
    if ($length <= 0) {
        return [];
    } elseif ($length == 1) {
        return array_merge($small, $capital);
    } else {
        $result = [];
        foreach (array_merge($small, $capital) as $letter) {
            foreach (generate($length - 1) as $subLetter) {
                $result[] = $letter . $subLetter;
            }
        }
        return $result;
    }
}

// Sample output
echo '<pre>' . print_r(generate(2), true) . '</pre>';