在php中传递contenteditable的内容到输入值和进程

Question

html code structure is as it follows a simple form

<form action="index.php" method="post" enctype="multipart/form-data">
  <div contenteditable="true" id="post"></div>
  <input type="text" name="postHiddenField" id="postHidden" hidden>
  <button type="submit" name="submitPost">Post</button>
</form>

beneath the form the javascript is a simple atribute

var content = document.getElementById('post').innerHTML;
document.getElementById('postHidden').value = content;

on top of the form is the form being proccessed

<?php
if (isset($_POST['submitPost'])) {
   if (!empty($_POST['postHiddenField'])) {
        echo $_POST['postHiddenField'];
   } else {
      echo "field is empty";
   }
}
?>

result i get on submission is field is empty how to proper get content's editable content and pass it to the hidden field as value?