Gmod - Custom Load Screen

Ok, so I know how to get the custom loading screen when joining a server on gmod.
I’ve been trying to follow this: but it has far to little information!

Here is what I put in my loadscreen.php

$authserver = bcsub( $communityid, '76561197960265728' ) & 1;
//Get the third number of the steamid
$authid = ( bcsub( $communityid, '76561197960265728' ) - $authserver ) / 2;
//Concatenate the STEAM_ prefix and the first number, which is always 0, as well as colons with the other two numbers
$steamid = "STEAM_0:$authserver:$authid";

and the URL in autoexec.cfg

"gamemode" "deathrun"
"sv_gamemode" "deathrun"
"dr_roundtime_seconds" "320"
"dr_highlight_admins" "1"
"dr_total_rounds" "8"
"dr_allow_death_suicide" "0"
"dr_allow_autojump" "1"
"dr_unlimited_ammo" "1"
"dr_allow_death_pickup" "1"
"dr_realistic_fall_damage" "1"
"dr_notify_rounds_left" "1"
"dr_push_collide" "0"
"dr_death_rate" "0.25"
"dr_death_max" "4"
host_workshop_collection 175028513 -authkey 450A166ED815F6378A10D417BAE654FD
sv_loadingurl ""

But I get this

$communityid is undefined

Then how do I define it?
I know very little PHP.

You got that code from this page:

so all you have to add is this to the top:
//Get the steamid (really the community id)
communityid = _GET[“steamid”];
//Get the map name
mapname = _GET[“mapname”];
//See if the second number in the steamid (the auth server) is 0 or 1. Odd is 1, even is 0

[editline]2nd September 2013[/editline]

If you are designing the web page, you can print the information like this (in your html):
<?php echo “Your SteamID is $steamid” ?>
and so on. The variable is $steamid which got defined earlier in your php code. Also, notice how i put <?php and wrote only php code inside, and closed it with ?>. Anything between those tags should be php code, but you can put those tags inside html.

function DownloadingFile( fileName ) {echo “downloading fileName”;} ?