Timed one time password

totp.php

<?php
$digits   = 6;
$interval = 30;
$secret   = 'secret_key';

$prevTotp = '';
for ($i = 0; $i < $interval * 4; $i++) {
    $time    = time();
    $counter = (string) floor($time / $interval);
    $hmac    = hash_hmac('sha1', $counter, $secret);
    $offset  = hexdec(substr($hmac, -1, 1));
    $otp     = hexdec(substr($hmac, $offset * 2, 8));
    $totp    = sprintf('%06d', $otp % 10 ** $digits);
    if ($totp !== $prevTotp) {
        $prevTotp = $totp;
        echo PHP_EOL;
    }

    printf("\r\e[2KTime: %d\tCounter: %d\tTOTP: %0{$digits}d", $time, $counter, $totp);
    sleep(1);
}
echo PHP_EOL;