Replace all characters of a string with asterisks except for the last four characters

Question

I want to mask the leading digits of a phone number using asterisks.

So that a message can be presented as:

Your phone number ********8898 has been sent with a verification code.

Answer 1

If you wanna mask middle part:

private function stringToSecret(string $string = NULL)
{
    if (!$string) {
        return NULL;
    }
    $length = strlen($string);
    $visibleCount = (int) round($length / 4);
    $hiddenCount = $length - ($visibleCount * 2);
    return substr($string, 0, $visibleCount) . str_repeat('*', $hiddenCount) . substr($string, ($visibleCount * -1), $visibleCount);
}

Result:

• 1 => 1
• 12 => 12
• 123 => 1*3
• 1234 => 1**4
• 12345 => 1***5
• 123456 => 12**56
• 1234567 => 12***67
• 12345678 => 12****78
• ...