SAP ABAP GUID22 in PHP

guid22.php

<?php
  
/*
  guid22 is base64 with a slightly changed alphabet (numbers first)
*/
function guid22(string $string): string {
  static $alphabet = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/';

  $length = strlen($string);

  if ($length <= 0) {
    return '';
  }

  $buffer = 0;
  $pos = 0;
  $result = '';

  for ($i = 0; $i < $length; $i++) {
    $buffer = ($buffer << 8) + ord($string[$i]);
    $pos += 8;
  
    while ($pos >= 6) {
      $pos -= 6;
      $result .= $alphabet[($buffer >> $pos) & 0x3f];
    }
  }

  if ($pos > 0) {
    $result .= $alphabet[$buffer << (6 - $pos) & 0x3f];
  }

  return str_pad($result, ceil(strlen($result) / 8) * 8, '=');
}