Please try using this CVS snapshot:

  http://snaps.php.net/php4-STABLE-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php4-win32-STABLE-latest.zip

With http://snaps.php.net/php4-STABLE-latest.tar.gz the error also occurs :(

What are your variables_order and gpc_order settings?
(from phpinfo() output)
Which webserver? Apache? What version?

variables_order = EGPCS
gpc_order = GPC
Apache Version = Apache/2.0.47 (Unix) mod_ssl/2.0.47 OpenSSL/0.9.6c DAV/2 PHP/4.3.3RC5-dev

What was the configure line used to configure Apache?
(And PHP too)

Apache-configure:
"./configure" \
"--prefix=/usr/local/apache2" \
"--with-mpm=prefork" \
"--enable-modules=deflate expires include info rewrite ssl unique_id usertrack log_config logio rewrite vhost_alias so ext_filter dav headers speling"

PHP-configure:
'./configure' '--with-mysql=/usr/local/mysql' '--enable-ftp' '--with-gd' '--with-gd-dir=/usr/local' '--with-jpeg' '--with-jpeg-dir=/usr/local' '--with-apxs2=/usr/local/apache2/bin/apxs' '--with-gettext' '--with-mcrypt' '--with-imap' '--with-imap-ssl' '--with-ldap' '--with-zlib-dir=/usr/include' '--enable-xslt' '--with-xslt-sablot' '--enable-exif'

One more thing: Where do you set "register_globals" ?
(php.ini and/or httpd.conf and/or .htaccess ?)

And can you try with this script instead:

<?php
    error_reporting(E_ALL);
    echo '<pre>';
    var_dump($HTTP_HOST);
    var_dump($_SERVER['HTTP_HOST']);
    echo '</pre>';
?>

I can not reproduce this..

I've created a Directory with a few interesting files:

    http://www.lansco.de/php/

Please try

    http://www.lansco.de/php/bug-25172.php

as often as necessary to reproduce the error.

"register_globals" is only set in the php.ini

I can reproduce the error with
- 4.3.2
- 4.3.3-rc4
- 4.3.3-rc5-dev

I cannot reproduce the error with
- 4.3.1

I run all the test-series on the same machine (www.lansco.de).

Use this command line to test it on my server:

  watch -n 1 "wget http://www.lansco.de/php/bug-25172.php -q -O xxx;cat xxx>>zzz;tail -n 30 zzz"

After a few minutes you can see the errors in the zzz-file.

Your test site shows the correct output all the time..
Have you checked phpinfo() output when this happens?
(add it to your test script)
What is in section "Apache Environment" for HTTP_HOST then?
(be sure the script gives the error about $HTTP_HOST being undefined)
What is register_globals setting set to at that time? (phpinfo(), both values)

I have tested it right now with this command line:

# watch -n 1 "wget http://www.lansco.de/php/bug-25172.php -q -O xxx;cat xxx>>zzz;tail -n 30 zzz"

For about 10 minutes no error occured. After that time in nearly every request I have had this error for about 30 times.

I added the line

  if(!isset($HTTP_HOST)) phpinfo();

to http://www.lansco.de/php/bug-25172.php and there is a really surprising phenomenon:
In the phpinfo()-output HTTP_HOST is correctly set to www.lansco.de!!!

register_globals is only set by the php.ini-file.

Please try again and test it for about 20 to 30 minutes.

Yes, it took about 30 minutes until I got the same result.
This is what I found:
register_globals Off On

Something caused this..are you ABSOLUTELY sure you don't use ANY "php_value" or "php_admin_value" or "php_flag" or "php_admin_flag" directives in httpd.conf / .htaccess files?
(In ANY virtual host you might have there)

I've found one (among thousands) .htaccess-File that contains "php_flag register_globals off".

When I access the site that has this entry simultaneous with my test page the error occurs. Elsewise not.

After I have disabled the .htaccess-entry the error doesn't occur any more.

I have tested again various versions of PHP while accessing the site with .htaccess-entry:

PHP-4.3.1: no errors
PHP-4.3.2: errors
PHP-4.3.3: errors

Why does it properly work with PHP-4.3.1 and not with later versions?

See bug #25753 (placeholder for all these reports about leaking php ini settings)

This should now be fixed in CVS. Fix scheduled for PHP 4.3.5.

I can report experiencing this problem under IIS6 and PHP 4.3.7.

It occurs when 'register_globals' is off.