$http_host use case

Hi all, I make 2 sites with 2 domain names that use same scripts, Each has got its html templates in a special dir selected judgging from $HTTP_HOST values. I was wondering me how a web crawler (search engine robot) could refer those pages. Would crawler bring that parameter in order to refer well both sites... I see a short explanation of http_host at : http://www.php.net/manual/en/reserved.variables.php#reserved.variables.serve r => 'HTTP_HOST' Contents of the Host: header from the current request, if there is one. It makes me doubt. My server is an apache php 1.3.20 with 1 ip so http 1.1 is mandatory, how search engine works, did they affect this var correctly ? Is there any crawlers that doesnt support http 1.1 again ? If there isn't this var my site would melt down ! merci. a+.