พอ login พร้อมกันหลายคนแล้ว ขึ้นแต่ข้อมูลของคนล่าสุด ประกาศเป็น session ไว้อ่าคะ ควรแก้ยังไงหรอคะ

<?php
// *** Validate request to login to this site.
if (!isset($_SESSION)) {
  session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
  $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['username_st_login'])) {
  $loginUsername=$_POST['username_st_login'];
  $password=$_POST['password'];
  $MM_fldUserAuthorization = "";
  $MM_redirectLoginSuccess = "st_apply_time.php";
  $MM_redirectLoginFailed = "st_login_fail.php";
  $MM_redirecttoReferrer = false;
  mysql_select_db($database_con_apply, $con_apply);
  
  $LoginRS__query=sprintf("SELECT p_card, p_phoneme FROM profile WHERE p_card=%s AND p_phoneme=%s",
    GetSQLValueString($loginUsername, "text"), GetSQLValueString($password, "text")); 
   
  $LoginRS = mysql_query($LoginRS__query, $con_apply) or die(mysql_error());
  $loginFoundUser = mysql_num_rows($LoginRS);
  if ($loginFoundUser) {
     $loginStrGroup = "";
    
    //declare two session variables and assign them
    $_SESSION['MM_Username'] = $loginUsername;
    $_SESSION['MM_UserGroup'] = $loginStrGroup;	      

    if (isset($_SESSION['PrevUrl']) && false) {
      $MM_redirectLoginSuccess = $_SESSION['PrevUrl'];	
    }
    header("Location: " . $MM_redirectLoginSuccess );
  }
  else {
    header("Location: ". $MM_redirectLoginFailed );
  }
}
?>