Format phone number which contains spaces and dashes into every three numbers separated by dashes

format_phone_number.php

public function format_phone_number($S) {
    $S = str_replace(' ', '', $S);
    $S = str_replace('-', '', $S);
    $len = strlen($S);
    $num_groups = floor($len / 3);
    $remain = $len % 3;

    if ($num_groups >= 1 && $remain == 1) {
        $num_groups -= 1;
        $remain += 3;   //4
    }

    if ($num_groups === 0) {
        return $S;
    }

    $str_result = '';
    $result = array();
    for ($i = 0; $i < $num_groups; $i++) {
        $index_min = $i * 3;
        $s_sub = substr($S, $index_min, 3);
        $result[] = $s_sub;
    }

    if ($remain !== 0) {
        $s_remain = substr($S, $num_groups * 3, $remain);
        if ($remain < 3) {
            $result[] = $s_remain;
        } else {
            $result[] = substr($s_remain, 0, 2);
            $result[] = substr($s_remain, 2, 2);
        }
    }

    $str_result = implode('-', $result);

    return $str_result;
}

Task:
A raw phone number might include spaces and dashes; Spaces are to be removed; and every three numbers are to be separated by one dash; The last segment can have 1 or 2 numbers; If there are 4 numbers contained in the two last segments, then two numbers are placed for each of the last two segments.