Array as session variable PHP7

[nicemotion]

Ho questo script di login tramite form dove tra le altre cose viene settata la var di sessione $_SESSION['MM_UserGroup'] :

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

if (isset($_POST['username'])) {
  $loginUsername=mysqli_real_escape_string($tao_conn, $_POST['username']);

  $password=sha1($_POST['password']);
 
 
  $MM_fldUserAuthorization = "AuthLev";
  $MM_redirectLoginSuccess = "Pub_LoginRedirect.php";
  $MM_redirectLoginFailed = "Pub_loginSorry.php";
  $MM_redirecttoReferrer = false;
  mysqli_select_db($tao_conn, $database_tao_conn);
  $LoginRS__query=sprintf("SELECT email01, Pwd, AuthLev FROM utenti WHERE email01=%s AND Pwd=%s",
  GetSQLValueString($tao_conn,$loginUsername, "text"), GetSQLValueString($tao_conn,$password, "text"));
   
  $LoginRS = mysqli_query($tao_conn,$LoginRS__query) or die(mysqli_error($tao_conn));
  $loginFoundUser = mysqli_num_rows($LoginRS);
  if ($loginFoundUser) {
    
  $loginStrGroup  = mysqli_result($LoginRS,0,'AuthLev');

    if (PHP_VERSION >= 5.1) {session_regenerate_id(true);} else {session_regenerate_id();}
    //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 );
  }
} 

Quando la richiamo nelle varie pagine

 echo $_SESSION['MM_UserGroup'] ; 

in php5 mi restituisce il valore ma in php7 restituisce "Array" con il seg errore:

Notice: Array to string conversion in ....