WOL Apgar routine in PHP

apgar.php

<?php  
echo apgar('password'); //result WaIMMsbf

function apgar($a) {  
  if (strlen($a) == 8) {
    $u = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    for ($i = 0; $i <= 7; $i++) {
      $r = ($i == 0) ? 0 : ord($a[8 - $i]);
      $x = (ord($a[$i]) & 1) ? (ord($a[$i]) << 1) & $r : ord($a[$i]) ^ $r;
      $o[] = substr($u, ($x & 0x3f), 1);
    }
    return implode('', $o);
  }
}