foo.php

<?php

// Alter these to your instance, do not share publicly like this.
$base_api_url = 'http://bookstack.local/api/';
$token_id='ZNI62nNoghS3GyfzThlZFXqYNtHdHnM6';
$token_secret='zPffPZ1ATQcv4Nz75QYLEbMm6fPzmo7I';

// Id of the shelf we want to add our book to.
$shelfId = 12; // $_POST["category"]

// Info for the new book
$bookInfo = [
    'name' => 'My new book', // $_POST['title']
    'description' => 'My new book description', // $_POST['description']
];

// Create our new book
$book = request('/books', 'POST', $bookInfo);

// Get the shelf and create an array of the existing book ids
$shelf = request("/shelves/{$shelfId}", "GET");
$shelfBookIds = [];
foreach ($shelf->books as $existingBook) {
    $shelfBookIds[] = $existingBook->id;
}

// Add our new book id to the array of book ids for the shelf
$shelfBookIds[] = $book->id;

// Update our shelf with the new books
$updatedShelf = request("/shelves/{$shelfId}", "PUT", [
    "books" => $shelfBookIds,
]);

echo '<h1>Vielen Dank</h1>';

/**
 * Make a request to BookStack and decode the response to JSON.
 * Will die on any curl errors.
 */
function request(string $path, string $method = 'GET', array $data = []): stdClass {
    global $base_api_url, $token_id, $token_secret;

    $url = rtrim($base_api_url, '/') . '/' . ltrim($path, '/');
    $curl = curl_init($url);

    curl_setopt($curl, CURLOPT_CUSTOMREQUEST, strtoupper($method));
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_HTTPHEADER, ["Authorization: Token {$token_id}:{$token_secret}"]);
    if (count($data) > 0) {
        curl_setopt($curl, CURLOPT_POSTFIELDS, http_build_query($data));
    }

    $response = curl_exec($curl);
    $err = curl_error($curl);
    curl_close($curl);
    if ($err) {
        die("Request to {$url} failed with error {$err}");
    }

    return json_decode($response);
}