left join query

<?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);
    $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";
      }
    endforeach;
  }
?>