如何使用Fetch API更新php $ _POST数组

Question

I am trying to send a form data to my php file using the fetch API but nothing work.

Here's my .js file

form.addEventListener("submit", e => {
       e.preventDefault();

  let message = document.getElementById("message").value,
    name = document.getElementById("name").value;

  let object = {
    name,
    message
  }
  //Apply encodeURIComponent to element
  function formEncode(obj) {
    var str = [];
    for (var p in obj)
      str.push(encodeURIComponent(p) + "=" + encodeURIComponent(obj[p]));
    return str.join("&");
  }

  fetch("http://localhost/essai/script.php", {
    method: "POST",
    mode: "same-origin",
    credentials: "same-origin", // For COOKIES
    headers : {
      'Content-type': ''
    },

    body: formEncode(object)
  })
    .then(response => response.text)
    .catch(err => console.log(`Une erreur s'est produite : ${err}`))
});

Here's my .php file

<?php
$contentType = isset($_SERVER["CONTENT_TYPE"]) ? trim($_SERVER["CONTENT_TYPE"]) : '';

if ($contentType === "application/json") {
  //Receive the RAW post data.
  $content = trim(file_get_contents("php://input"));

  $decoded = json_decode($content, true);

  //If json_decode failed, the JSON is invalid.
  if(! is_array($decoded)) {
    echo $_GET['name'];
  } else {
    // Send error back to user.
    echo "Une erreur s'est produite";
  }
}

the echo $_POST['name'] don't display the user name added in the input , Please help me solve this . Thank You !