title: $_POST empty after fetch formData
body: <p>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?</p>
  
  <pre><code>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);
  </code></pre>
  
  <p>.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.</p>
  
  <pre><code>$conn=new mysqli($servername, $username, $password, $dbname);
  if($conn-&gt;connect_error)
  {
      die ("Connection failed " .$conn-&gt;connect_error);
  }
  
  if(isset($_POST["titolo_racc"]) &amp;&amp; isset($_POST["id_google"] &amp;&amp; isset($_POST["titolo"] &amp;&amp; isset($_POST["autore"] &amp;&amp; isset($_POST["editore"] &amp;&amp; isset($_POST["pagine"]))
  {
      print_r($_POST); // array ()
      print_r($_POST["titolo"]; //Undefined index:
  }
  </code></pre>