This tutorial was created for an older version of PHP.
PHP 8.3 produces warnings about using dynamic properties:
Deprecated: Creation of dynamic property ... is deprecated in ... on line ...
That disrupts the JSON output. You will see this warning in HTTP response if you check the Network tab in browser developer tools.
To fix this issue, you need to review the PHP scripts that produce this warning and modify the class definitions to include the properties.
Before:
class Event {}
After:
class Event {
public $id;
public $text;
public $start;
public $end;
public $resource;
}