Very good points above on seeds, thank you. If you would like to test a seed try using the code below. It will take between 5 and 20 seconds depending on your system and then will spit out the number of reused keys out of 100,000 attempts.

;  for ($i=0; $i<100000; $i++) {
;    mt_srand(hexdec(substr(md5(microtime()), -8)) & 0x7fffffff);
;    $rand = mt_rand();
;
;    ($arr[$rand] == '1') ? $k++ : $arr[$rand] = '1';
;  }