Mine

<?php

class mac_gen {
  public $hex;
  public function __construct() {
    $this->hex = str_split('ABCDEF0123456789');
    shuffle($this->hex);
    $this->hex = array_flip($this->hex);
  }
  public function main() {
    return '00' . ':' . implode(array_rand($this->hex, 2)) . ':' . implode(array_rand($this->hex, 2)) . ':' . implode(array_rand($this->hex, 2)) . ':' . implode(array_rand($this->hex, 2)) . ':' . implode(array_rand($this->hex, 2));
  }
}

$mac = new mac_gen;

while(true) {
  usleep(0);
  echo $mac->main() . "\n";
}
?>