this example (of the documentation of mt_srand()) 
<?
      // seed with microseconds since last "whole" second
      mt_srand ((double) microtime() * 1000000);
      echo mt_rand();
?>
always give the same value.

this example work well
<?
      // seed with microseconds since last "whole" second
      mt_srand (substr(microtime(),2));
      echo mt_rand();
?>