<?php
session_start();

header('Content-type: text/plain; charset=utf-8');
if (empty($_SESSION['valami'])) {
  $_SESSION['valami'] = 'queryString: '. $_SERVER['QUERY_STRING'];
  echo 'Ahol vagyok: ', $_SESSION['valami'];
}
else {
  echo 'Ahol voltam: ', $_SESSION['valami'];
  unset($_SESSION['valami']);
}