fulcrm_webhook/fulcrm_webhook.module

<?php

function fulcrm_webhook_permission() {
  return array( 'administer fulcrm webhooks' => array( 'title' => t('Administer fulcrm webhooks'),
                                                       'description' => t('Perform main installation/administration tasks for fulcrm webhooks.'),
                                                       ),                          
                );
}

function fulcrm_webhook_menu() {
    /*
    $items[ 'admin/config/services/fulcrm' ] = array( 'page callback' => 'system_admin_menu_block_page',
                                                      'file' => 'system.admin.inc',
                                                      'file path' => drupal_get_path('module', 'system'),
                                                      'title' => 'fulcrm',
                                                      'description' => 'Configuration integration with fulcrm.org.',
                                                      'position' => 'right',
                                                      'access callback' => 'user_access',
                                                      'access arguments' => array('access administration pages'),
                                                      );
   */
    $items[ 'admin/config/services/fulcrm/webhook' ] = array( 'page callback' => 'fulcrm_webhook_admin',
                                                              'file' => 'fulcrm_webhook.admin.inc',
                                                              'title' => 'fulcrm webhooks',
                                                              'description' => 'Configure webhooks for fulcrm.org.',
                                                              'access callback' => 'user_access',
                                                              'access arguments' => array('administer fulcrm webhooks'),
                                                              );
    $items[ 'services/fulcrm/webhook/%' ] = array( 'page callback' => 'fulcrm_webhook_webhook',
                                                   'page arguments' => array(3),
                                                   'type' => MENU_CALLBACK,
                                                   'access callback' => 'user_access',
                                                   'access arguments' => array('access content'),
                                                   );

    return $items;
}

function fulcrm_webhook_webhook( $uuid ) {
    if ( $uuid === variable_get( 'fulcrm_webhook_url' ) ) {
        $headers = getallheaders();
        $payload = null;
        $payload_error = null;
        $method = $_SERVER[ 'REQUEST_METHOD' ];

        if ( array_key_exists( 'Content-Type', $headers ) && ( $headers[ 'Content-Type' ] === 'application/json' ) ) {
            $payload = json_decode( file_get_contents( "php://input" ) );
            $payload_error = json_last_error();
            $payload_error_msg = json_last_error_msg();
        }

        $client_id = variable_get( 'fulcrm_apiv2_client_id' );

        if ( fulcrm_apiv2_prevent_loop( $headers ) ) {
            switch ( $method ) {
            case 'GET':
                drupal_json_output( array( "status" => "error", "info" => "GET requests do nothing; webhooks must be POST, PATCH, PUT, or DELETE" ) );
                break;
            case 'DELETE':
            case 'POST':
            case 'PATCH':
            case 'PUT':
                if ( $payload === null ) {
                    drupal_json_output( array( 'status' => 'error', 'info' => $payload_error_msg ) );
                } else {
                    // actually do something :)
                    drupal_json_output( array( 'status' => 'ok', 'headers' => $headers, 'method' => $method ) );
                }
                break;
            default:
                drupal_json_output( array( 'status' => 'error', 'info' => 'unsupported method' ) );
                break;
            }
        } else {
            drupal_json_output( array( 'status' => 'ok', 'info' => 'loop prevention' ) );
        }
    } else {
        drupal_access_denied();
    }
}