foo.php

<?php
$service_url = 'yourdomain.org/api/books';
$curl = curl_init($service_url);

$curl_post_data = array(
    'name' => $_POST['title'],
    'description' => $_POST['description']
);


curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_HTTPHEADER, array(
    'Authorization: Token '
));
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_POSTFIELDS, $curl_post_data);
$curl_response = curl_exec($curl);
if ($curl_response === false) {
    $info = curl_getinfo($curl);
    curl_close($curl);
    die('error occured during curl exec. Additional info: ' . var_export($info));
}
curl_close($curl);

$decoded = (array) json_decode($curl_response, true); 
echo($decoded["id"]);
echo('</br>');
echo($_POST["category"]);
process_book($decoded["id"], $_POST["category"]);

if (isset($decoded->response->status) && $decoded->response->status == 'ERROR') {
    die('error occured: ' . $decoded->response->errormessage);
}
echo '<h1>Vielen Dank</h1>';

var_export($decoded->response);


function process_book($id, $category) {

    $id = strval($id);
    $url = 'yourdomain.org/api/shelves/' . '{' .$id .'}';
    $curl = curl_init($url);
    
    $update_date = "'books': [$id]";

    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "PUT");    
    curl_setopt($curl, CURLOPT_HTTPHEADER, array(
        'Authorization: Token '
    ));

    curl_setopt($curl, CURLOPT_POSTFIELDS, $update_date);

    $shelf_list = curl_exec($curl);

    if ($shelf_list === false) {
        $info = curl_getinfo($curl);
        curl_close($curl);
        die('error occured during curl exec. Additional info: ' . var_export($info));
    }
    if (isset($decoded->response->status) && $decoded->response->status == 'ERROR') {
        die('error occured: ' . $decoded->response->errormessage);
    }
    curl_close($curl);
}
?>
`