if($USER["user_id"]){
    echo "logged in";
} else {
    echo "logged out";
}

if($GLOBALS['USER']['user_id']){
    echo "logged in";
} else {
    echo "logged out";
}

<?php

HERE print "<p>quick_auth.php loaded<br />"; #DEBUG LINE
if(isset($_COOKIE["phorum_session_v5"])){
HERE print "_COOKIE[\"phorum_session_v5\"] set<br />"; #DEBUG LINE
    if(isset($PHORUM["user"])){
print "_PHORUM[\"user\"] set<br />"; #DEBUG LINE

        $GLOBALS["USER"] = $PHORUM["user"];

    } else {

HERE print "_PHORUM[\"user\"] not set<br />"; #DEBUG LINE
        define("PHORUM", "quick_auth");

        // wrap all this in a function to protect it and other code
        function check_session()
        {
            // one of these needs to be commented out.
            $require = "sessid_lt";  // long term cookie that will exist over time
            //$require = "sessid_st"; // short term cookie that may not be required at all, but is more secure
HERE print "Require: $require<br />";
            include_once($_SERVER["DOCUMENT_ROOT"]."/board/include/db/config.php");
            list( $user_id, $sess_id ) = explode( ":", $_COOKIE["phorum_session_v5"], 2 );

            $conn=mysql_connect($PHORUM["DBCONFIG"]["server"], $PHORUM["DBCONFIG"]["user"], $PHORUM["DBCONFIG"]["password"], true);
            mysql_select_db($PHORUM["DBCONFIG"]["name"], $conn);

            $sql="select * from ".$PHORUM["DBCONFIG"]["table_prefix"]."_users where user_id='".mysql_escape_string($user_id)."' and $require='".mysql_escape_string($sess_id)."'";

            $res=mysql_query($sql, $conn);

            if(mysql_num_rows($res)){
HERE print "Results found for ".$PHORUM["DBCONFIG"]["table_prefix"]."_users where user_id='".mysql_escape_string($user_id)."<br />"; #DEBUG LINE
                $USER=mysql_fetch_assoc($res);

                $sql="select data from ".$PHORUM["DBCONFIG"]["table_prefix"]."_settings where name='PROFILE_FIELDS'";

                $res=mysql_query($sql, $conn);
                if(mysql_num_rows($res)){
HERE print "Data found for ".$PHORUM["DBCONFIG"]["table_prefix"]."_settings where name='PROFILE_FIELDS' <br />"; #DEBUG LINE
                    $fields=unserialize(mysql_result($res, 0, "data"));

                    $sql="select * from ".$PHORUM["DBCONFIG"]["table_prefix"]."_user_custom_fields where user_id='".mysql_escape_string($user_id)."'";

                    $res=mysql_query($sql, $conn);
                    if(mysql_num_rows($res)){
                        while($row=mysql_fetch_assoc($res)){
HERE print "Row data: ".$row["data"]."<br />";
                            $USER[$fields[$row["type"]]["name"]]=$row["data"];
                        }
                    }
                }

            } else {
print "No user found<br />"; #DEBUG LINE
                $USER=array();
            }

            $GLOBALS["USER"]=$USER;
HERE print "GLOBALS['USER']: ".$GLOBALS["USER"]."<br />";
        }


        check_session();
    }
}


?>

require_once("/home/dcshows/www/lib/quick_auth.php");

if($USER["user_id"]){
    echo "logged in";
} else {
    echo "logged out";
}

• You are accessing the forum on a different domain / hostname than the main site;
• Phorum is inside a subdirectory and the cookie path in the general settings was changed to reflect that;
• Other reasons that I didn't think of.

<?php
if ( isset($GLOBALS['USER']) && !empty($GLOBALS['USER']['user_id']) ) {
    $display_name = htmlspecialchars($GLOBALS['USER']['display_name']);
    print "Hello, $display_name!";
} else {
    print "Hello, world!";
}
?>