Need advice for testing in php

I'm very new to tests. I have a webapp that stores books and informations about books.

If i call my route /store/ it trigger my store methods and creating a new book into my database. This are the simplified methods:

Book.php class file:

public function __construct() {
  $this->bookService = new BookService();
}

public function store() {
    $input = /* get post input values */
    $this->createDatabaseEntry($input);
}

private function createDatabaseEntry($input) {
    // Creating database entry
    $book = /* bla bla */

    $languages = $this->bookService->fetchLanguages($book->goodreads_id);
    // And here i loop over the $languages and store them all in a extra table.
}

And here i have an other Service class BookService.php:

public function fetchLanguages($goodreads_id) {
    // Here i make an guzzle http call to a other service from goodreads.com
}

How do i test this without making a http request to goodreads? I need to verify the languages are in the database. I can work with fixtures for fetchLanguages(). But how can i stub/mock (i don't know the correct term) this function?

I using Laravel and PHPUnit.