The function mt_rand() has a "preference" for the value 1898087491 for a majority (but not all) of the default seedings.  As a result, if you don't seed it you will generate far too many collisions on the value 1898087491.  

If you seed it with values such as 0, 1, 2, 3, etc., the problem will not occur (these are all the seeds that I tried); however, the mt_rand should work properly when not seeded.  Further, there SHOULD NOT be any single seed that causes this value to occur more than others.   This leads me to believe that mt_rand() is not implemented correctly when not seeded.

Here it says that it is not necessary to use mt_srand() to obtain "random" data:
http://us3.php.net/manual/en/function.mt-srand.php

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The following shows a test script that reproduces the problem:

D:\php4-win32-STABLE-200306052030>type unseeded2.php

<? 
for ($i = 0; $i < 25; $i++) 
  print `php -q unseeded.php`."\n"; 
?>

D:\php4-win32-STABLE-200306052030>type unseeded.php

<?
        $j = 0;
        for ($i = 0; $i < 65536; $i++)
                if (mt_rand() == 1898087491) $j++;
        print $j;
?>

D:\php4-win32-STABLE-200306052030>php unseeded2.php
Content-type: text/html
X-Powered-By: PHP/4.3.3-dev

36480
39710
2308
41576
38524
39247
5005
5449
38011
29808
37577
5738
46785
42418
45619
42322
39296
15909
35019
39127
26830
37223
33348
22713
0