You can't. Well, not always.

IF it is available, you'll find it in the REMOTE_HOST environment
variable.  However, this will more often than not contain the numerical
IP address rather than the IP name of the remote host. Remember that
not all IP addresses have a hostname associated with them; this is the
case of most IP addresses assigned to dialup users, for example. Your
web server may also not perform a reverse lookup on incoming
connections, in which case REMOTE_HOST will contain the IP address even
if it has a corresponding IP name. In the second case, you can do a
reverse lookup yourself in your script, but this is expensive and
should probably be avoided unless absolutely necessary.

Even if you do manage to obtain a hostname, you should be aware that it
may not correspond to the hostname the user is accessing your page
from. It may instead be that of an intervening proxy host.

The short answer is therefore that there is no reliable way of finding
out what the remote user's hostname is.