Simple PHP implementation of Nanoid, secure URL-friendly unique ID generator

nanoid.php

<?php

/**
 * Generate secure URL-friendly unique ID.
 * By default, ID will have 21 symbols to have same collisions probability
 * as UUID v4.
 *
 * @param integer $len The length of the ID to generate.
 * @param mixed $entropy balanced, true (max), false (min)
 * @param string $alphabet The characters to use in the ID.
 *
 * @return string random string of characters
 */
function generateNanoId(int $len = 21, mixed $entropy = 'balanced', string $alphabet = '_-0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'): string
{
    $id = '';
    $alphabet_len = strlen($alphabet);

    if ($entropy === false || $entropy == 'balanced') {
      $tolen = $entropy == 'balanced' ? $len / 2 : $len;
      for ($i = 0; $i < $tolen; $i++) {
        $id .= $alphabet[random_int(0, $alphabet_len - 1)];
      }
      if ($entropy != 'balanced') {
        return $id;
      }
    }

    $mask = (2 << (int) (log($alphabet_len - 1) / M_LN2)) - 1;
    $step = (int) ceil(1.6 * $mask * $len / $alphabet_len);

    while (true) {
      $bytes = unpack('C*', random_bytes($len));
      for ($i = 1; $i <= $step; $i++) {
        $byte = $bytes[$i]&$mask;

        if (isset($alphabet[$byte])) {
          $id .= $alphabet[$byte];

          if (strlen($id) === $len) {
            return $id;
          }
        }
      }
    }
}

// usage:
$nanoId = generateNanoId();
?>