İnternetten 30 saniyede buldum

function print_combinations($characters, $length, $combination = '') {
if ($length > 0) {
foreach ($characters as $i) {
print_combinations($characters, $length - 1, $combination . $i);
}
} else {
printf("%sn", $combination);
}
}

$characters = range(0, 9);
print_combinations($characters, 3);