Drupal 7 Ajax GET - Error 400 (Hook_menu)
The % symbol is type of placeholder in hook_menu item. You can read more about it here: https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_menu/7#sec_path_wildcards.

For you example I suggest to use the following:

function my_db_module_menu() {
  $items['callback'] = array(
    'title' =&gt; 'My callback',
    'description' =&gt; 'Desc',
    'page callback' =&gt; 'mymodule_getinfo',
    'type' =&gt; MENU_CALLBACK,
    'access arguments' =&gt; array('access content'),
  );

  return $items;
}


Simple path  - callback.

And use the following AJAX:

$.ajax({
  url: '/callback',
  type: 'POST',
  dataType: 'json',
  data: {data: post},
  ...


By this way, you can access the data in your webhook more simply:

/**
 * My callback
 */
function mymodule_getinfo() {
  $data = !empty($_POST['data']) ? $_POST['data'] : array();

  // do what you want...
}