This function is used to determine the IP address or DNS name of the calling client.
Please note that this function is slow when resolving a DNS names. It is advisable to use IP addresses to to make applications faster.
By default, a string of the dotted IP address of the client user agent. If what=1 then the DNS name will be returned.
http, http_value, http_url, string_output, http_acl_get, http_rewrite.