PHP 4.0.5 and 4.0.4pl1 (tried the latter as a fallback) downloaded as binaries from your php.net site, running Xitami.  Also tried on a Windows ME machine and got the same results, which are:

POSTing from a form does not result in the $HTTP_POST_VARS array being created.  It also doesn't create the global versions of these variables (with register_globals on or off).  However, it _does_ create appropriate entries in $HTTP_ENV_VARS['FORM_*'] and $HTTP_SERVER_VARS['FORM_*'].  Additionally, $HTTP_COOKIE_VARS show up fine (unrelated, just FYI).

I used this code to "emulate" (badly) the HTTP_POST_VARS variables, just to be sure that was really the problem:

if(count($HTTP_POST_VARS) < 2) { // test always succeeds
  reset($HTTP_ENV_VARS);         // is set, but is 0 size
  while(list($k,$v) = each($HTTP_ENV_VARS)) {
    if(!strncmp($k,"FORM_",5)) {
      $g = strtolower(substr($k,5));
      $HTTP_POST_VARS[$g] = $v;  // create fake array var
      $$g = $v;                  // create fake global var
    }
  }
}

I tried several scripts from the internet (Phorum, etc.) and had the same problem with all, whenever a form POSTed, on both machines using both versions of PHP.  Even a form with only a simple submit button fails without the above hack.  This is bizaare.

Here are some typical results from phpinfo() after a POST:

This is from the Environment section:

QUERY_METHOD POST 
REQUEST_METHOD POST 
 ...
FORM_SET Login 
FORM_AUTHKEY   
FORM_AUTHPASS 22222222 
FORM_AUTHUSER Myadmin23 
FORM_REFERERURL http://forum.hectorplasmic.com/frontpage.php 

This is from the PHP Variables section:

HTTP_SERVER_VARS["QUERY_METHOD"] POST 
HTTP_SERVER_VARS["REQUEST_METHOD"] POST 
 ...
HTTP_SERVER_VARS["FORM_SET"] Login 
HTTP_SERVER_VARS["FORM_AUTHKEY"]  
HTTP_SERVER_VARS["FORM_AUTHPASS"] 22222222 
HTTP_SERVER_VARS["FORM_AUTHUSER"] Myadmin23 
HTTP_SERVER_VARS["FORM_REFERERURL"] http://forum.hectorplasmic.com/frontpage.php 

So is this:

HTTP_ENV_VARS["QUERY_METHOD"] POST 
HTTP_ENV_VARS["REQUEST_METHOD"] POST 
HTTP_ENV_VARS["GATEWAY_INTERFACE"] CGI/1.1 
HTTP_ENV_VARS["SERVER_SECURITY"] - 
HTTP_ENV_VARS["SERVER_PROTOCOL"] HTTP/1.1 
HTTP_ENV_VARS["SERVER_PORT"] 80 
 ...
HTTP_ENV_VARS["SERVER_VERSION"] 2.5b1 
HTTP_ENV_VARS["SERVER_SOFTWARE"] Xitami 
HTTP_ENV_VARS["FORM_SET"] Login 
HTTP_ENV_VARS["FORM_AUTHKEY"]  
HTTP_ENV_VARS["FORM_AUTHPASS"] 22222222 
HTTP_ENV_VARS["FORM_AUTHUSER"] Myadmin23 
HTTP_ENV_VARS["FORM_REFERERURL"] http://forum.hectorplasmic.com/frontpage.php 


So is this, but they're my "fake" variables created by the code above.  These don't show up without that code -- no HTTP_POST_VARS at all.

HTTP_POST_VARS["set"] Login 
HTTP_POST_VARS["authkey"]  
HTTP_POST_VARS["authpass"] 22222222 
HTTP_POST_VARS["authuser"] Myadmin23 
HTTP_POST_VARS["refererurl"] http://forum.hectorplasmic.com/frontpage.php 

Here's the form (stripped of HTML formatting crud):

<form method="post" action="/login.php" name="login">
 <input type="hidden" name="refererurl" value="http://forum.hectorplasmic.com/frontpage.php"><br>
 Username:  <input class="inp" type="text" name="authUser">
 Userpass:  <input class="inp" type="password" name="authPass">
 Activation code:  <input class="inp" type="text" name="authKey">
 <input class="button" type="submit" name="set" value="Login">
 <input class="button" type="reset" name="reset" value="Clear">
</form>

I beat my head on the wall trying to figure it out -- obviously, this hasn't been happening to very many people, or you'd be flooded with complaints -- but I give up, I'm whupped.  Any ideas?

After I get some sleep, I may try PHP 3.x just to see if the problem remains...