[WP] Template Name: Check HTTP Referrer

<?php
/*
Template Name: Check HTTP Referrer
Description: Page template with referrer tracking by hostname. Allowed hostnames are defined in a custom field named 'allowed-hosts'. Use full hostnames (e.g. www.facebook.com vs facebook.com) and separate multiple values with a space. The template works as a normal page template if there's no custom field set or if 'track-referrer' is not set.
*/


// set vars
// HTTP referrer
$referrer = isset($_SERVER['HTTP_REFERER']) ? $_SERVER['HTTP_REFERER'] : false;

// get array of allowed hosts from custom field
$allowed_hosts = get_post_meta( $post->ID, 'allowed-hosts', true );
$allowed_hosts = !empty($allowed_hosts) ? explode( ' ', $allowed_hosts ) : false;

// custom field switch to turn tracking on/off
$tracking = get_post_meta( $post->ID, 'track-referrer', true );
$tracking = ($tracking == '1') ? true : false;

// error message and status
$error_msg = 'Not allowed.';

// debug
if(is_user_logged_in()) {
    $debug = '<pre>';
    $debug .= '<br />Enabled: ' . var_export($tracking, true);
    $debug .= '<br />Referrer: ' . var_export($referrer, true);
    $debug .= '<br />Host: ' . var_export(parse_url( $referrer, PHP_URL_HOST ), true);
    $debug .= '<br />Allowed hosts: ' . var_export($allowed_hosts, true);
    $debug .= '</pre>';

    $error_msg .= $debug;

    echo $debug;

}


// if referrer is set and custom fields exist
if( $referrer && $allowed_hosts && $tracking ) {

    // get hostname from referrer
    $host = parse_url( $referrer, PHP_URL_HOST );

    // add current page to array of allowed hosts because of form validation/submission
    $allowed_hosts[] = parse_url(get_permalink(), PHP_URL_HOST);

    // compare referrer to array of allowed hosts
    if ( in_array ( $host, $allowed_hosts, true ) ) {

        // load page template
        locate_template( array('page.php'), true, true);

    // referrer is not allowed
    } else {

        // error message and status
        wp_die($error_msg, '', array( 'response' => 403 ) );

    }

// no referrer but custom fields exist
} elseif ( !$referrer && $allowed_hosts && $tracking ) {

    // error message and status
    wp_die($error_msg, '', array( 'response' => 403 ) );

// no custom field (disable referrer tracking)
} else {

    // load page template
    locate_template( array('page.php'), true, true);

}