$user_age = 15; if ($user_age < 18) { trigger_error("Sorry, you are not old enough to access this website", E_USER_WARNING); // Generates a user-level warning error message }In this example, if the user's age is less than 18, a user-level warning error message will be generated using the `trigger_error` function. The error message will read "Sorry, you are not old enough to access this website", and the error level will be set to `E_USER_WARNING`. The PHP package library that includes the `trigger_error` function is the PHP core package. No additional library or package is required to use this function.