ketahuilah logic session

<?php
session_start();
if(isset($_POST['is_ajax']) && $_POST['is_ajax'])  {
    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);

    include "_class/fpldb.php";
    $logins    = new fpldb();
    $logselect = "users left join posit on users.id_posit = posit.id_posit";
    $logfield  = "users.username, users.password, posit.nm_posit";
    $logwhere  = "users.username ='".$username."' AND users.password ='".$password."'";
    $logselque = $logins->select($logselect,$logfield);
var_dump( $logselque);
echo "<br>select:$logselect |field:$logfield |where:$logwhere";
echo "<h3>btw wherenya kok gk kepake ya??</h3>";
    $logselres = $logins->getResult();

    foreach($logselres as $arrlog) :
      $_SESSION['username'] = $arrlog['username'];
      $_SESSION['password'] = $arrlog['password'];
      $_SESSION['position'] = $arrlog['nm_posit'];
      if($username == $_SESSION['username'] && $password == $_SESSION['password'] && $position = $_SESSION['position'])
      {
        echo "success";
      }else{
        echo "<br>session baru jalan kalau reload halaman bukan saat ini ";
      }
    endforeach;
  }else{
    echo 'tidak ada ajax disini';
}
?>