You could use something like this...
Code:
<?
print "<html>\n";
print "<head><title>Your IP Address</title></head>\n";
print "<body bgcolor=#FFFFFF>\n";
print "<h2 align=center>Computer Information</h2>\n";
print "<hr noshade width=50%>\n";
print "<table border=0 cellpadding=2 cellspacing=2 align=center>\n";
print "<tr>\n";
print " <td align=right>IP Address:\n";
print " <td align=left>$REMOTE_ADDR</td>\n";
print "</tr>\n<tr>\n";
print " <td align=right>Hostname:</td>\n";
print " <td align=left>" . gethostbyaddr($REMOTE_ADDR) . "</td>\n";
print "</tr>\n</table>\n";
print "<hr noshade width=50%>\n";
print "</body>\n";
print "</html>\n";
?>