If the whois is run via a php script, then it's your server's IP that will be involved not the user. The various whois servers have different rules. So you will need to check with the whois server owners site to see what those rules are. Some whois servers don't like too many requests. Some use systems to tell real people form automated requests. Others actually encorrage automated requests. If you are doing allot of requests, you could always setup your own local mirror so the searches are done locally.