获取formData后$ _POST为空

Question

when I try to fetch a formData with POST from JS to PHP, I obtain an empty $_POST array(), despite of Status Code is 200 OK. Can you help me?

function select_collection()
 {
  const tendina=document.querySelectorAll(".tendina");
   for(all of tendina)
        {
           all.addEventListener('change', aggiungi);
        }
    }
    function aggiungi(event){
        const tendina=event.target;
        const val=tendina.options[tendina.selectedIndex].value; 

        const row_box= tendina.parentNode.parentNode; 
        const child=row_box.childNodes;

            const url_image=child[0].childNodes[0].currentSrc;

            const id_google=child[1].childNodes[0].textContent.slice(11,);
            const titolo= child[1].childNodes[1].textContent.slice(8,);
            const autore= child[1].childNodes[2].textContent.slice(8,);
            const editore=child[1].childNodes[3].textContent.slice(9,);
            const pagine=child[1].childNodes[4].textContent.slice(8,);
            var formData = new FormData();
            formData.append('titolo_racc', val);
            formData.append('id_google', id_google);
            formData.append('titolo', titolo);
            formData.append('autore', autore);
            formData.append('editore', editore);
            formData.append('pagine', pagine);
            fetch('http://localhost/progetto/inserisci_contenuto.php',{
            method: "POST",
            body: formData
          }).then(onResponse).then(onText);

.tendina it's the class of a drop-down menù (select, option), title, author, publisher and pages refers to a book volume infos. I want to send this info to a PHP page.

$conn=new mysqli($servername, $username, $password, $dbname);
if($conn->connect_error)
{
    die ("Connection failed " .$conn->connect_error);
}

if(isset($_POST["titolo_racc"]) && isset($_POST["id_google"] && isset($_POST["titolo"] && isset($_POST["autore"] && isset($_POST["editore"] && isset($_POST["pagine"]))
{
    print_r($_POST); // array ()
    print_r($_POST["titolo"]; //Undefined index:
}

Answer 1

Get rid of:

    headers: new Headers({
        "Content-Type": "application/json;"}),

it's incorrect, since you're sending multipart/form-data format. You need to allow fetch() to provide this header by itself, since it needs to include the separator string, which is generated randomly internally.