Javascript

  var Form = document.createElement('form');
  Form.method = 'POST';
  Form.action = window.location.href;

  var Input = document.createElement('input');
  Input.type = 'hidden';
  Input.name = 'MyField';
  Input.value = MyValue;
  Form.appendChild(Input);

  document.body.appendChild(Form);
  Form.submit();

PHP

  if ( ($_SERVER['REQUEST_METHOD'] === 'POST') && (isset($_POST['MyField'])) )
  {
    $MyField= trim($_POST['MyField']);

  }