Erreur fetchAll() on a non-objet

<?php
//-------------------------------------------------------------------------//
//  Fichier de class : pdoFunc.class.php
// pour la connexion/gestion de la BDD  en PDO mysql
// autheur : Jordane  - Mai 2014
//-------------------------------------------------------------------------//


class pdoFunc extends PDO{
  //put your code here
  private $_obj_pdo,
          $_obj_pdo_prepare;
 
  public $_last_insert_id="";
  public $_query = array(); // contient la requête et les datas
  
  
 function __construct(){
   parent::__construct(); 
   //connexion à la BDD
   $this->cnx();
 }
 
 /**
 // Connnexion
 */
 public function cnx(){
   try{
    $_obj_pdo = new PDO('mysql:host=localhost; dbname=mabdd; charset=utf8', 'user', 'password');
    $_obj_pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); //activation des erreurs
    $_obj_pdo->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC); // type de fetch par défaut
    $this->_obj_pdo = $_obj_pdo;
  } catch(PDOException $e) {
	 $this->_obj_pdo = null;
     die('Erreur : ' . $e->getMessage());
	 
  }
}


/---------------------------------------------------------------------------------------------------------------------------//	
//  FONCTIONS
//---------------------------------------------------------------------------------------------------------------------------//	
		
 /**
 * Requête : SELECT
 * return : Array
 * fetchAll — > Retourne un tableau contenant toutes les lignes du jeu d'enregistrements 
 */
 public function querySelect($sql,$a_data=NULL){
   
  $this->_query = array('SQL'=>$sql, "A_DATA"=>$a_data);
  try{
   $this->_obj_pdo_prepare = $this->_obj_pdo->prepare($sql);
   $this->_obj_pdo_prepare->execute($a_data);  
   return $this->_obj_pdo_prepare->fetchAll();
  } catch(Exception $e) {
     echo "<pre><br>".$e->getMessage()."<br>";
     print_r($this->_query);
     echo "</pre>";
  }
 } 

 /**
 * Requête : INSERT
 * return : int : ID INSERT (Numéro de l'id auto_incrémenté créé lors de l'insert)
 */
 public function queryInsert($sql,$a_data=NULL){

  $this->_query = array('SQL'=>$sql, "A_DATA"=>$a_data);
    
  try{
     $this->_obj_pdo_prepare = $this->_obj_pdo->prepare($sql);
     $this->_obj_pdo_prepare->execute($a_data);
     $this->_last_insert_id = $this->_obj_pdo->lastInsertId();
     return $this->_last_insert_id ;
  } catch(Exception $e){
     echo "<pre>";
     print_r($this->_query);
     echo "</pre>";
     echo "<br>".$e->getMessage()."<br>";
  }
    
 }
  
 /**
 * Requête : EXEC (pour les Update/Delete..)
 * return : True/False
 */ 
 public function queryExec($sql,$a_data=NULL){
  $this->_query = array('SQL'=>$sql, "A_DATA"=>$a_data);
  try{
    $this->_obj_pdo_prepare = $this->_obj_pdo->prepare($sql);
    
    $result['RESULT'] = $this->_obj_pdo_prepare->execute($a_data);
    $result['NB_ROWS_AFFECTED'] = $this->_obj_pdo_prepare->rowCount();
    return $result; 
  } catch(Exception $e) {
    echo "<pre><br>".$e->getMessage()."<br>";
    print_r($this->_query);	
    echo "</pre>";
  }
 } 
 

 
}//----------- FIN DE LA CLASS -------------------//

<?php
//-----------------------------------------------------------------//
// Fichier users.class.php
// Class de gestion des users
// Necessite la class pdoFunc
//-----------------------------------------------------------------//
class users extends pdoFunc{
	private $table = "t_InfoJoueur";
	
	function __construct(){
		parent::__construct();
		
	}
	
	
 public Function GetUsers(){
 $result = NULL;
 $json = array();
 $response = $this->querySelect("SELECT * FROM $this->table");
 if(!empty($response)){
   $json = $response[0];
   $result = json_encode($json);
 }
 return $result;
}


public function userExist($user){
 $sql = "SELECT * FROM $this->table WHERE Pseudo_ID = :user":
 $datas = array(":user"=>$user);
 $res = $this->queryExec($sql,$datas);
 $result = !empty($res)? true : false;
 return $result;
}
	
	
	
public function ban($user)	{	
 if(!$this->userExist($user)){
  $result = "ERROR:USER_NOT_FOUND";
 }else{
  $this->queryExec("UPDATE $this->table SET Bani = 1 WHERE Pseudo_ID = ?");
  $result = "OK:BANNED";
 }
 return $result;
}	
	
		
// Fin de la class
}

require_once "pdoFunc.class.php";
require_once "users.class.php";

 $oUser = new users();

 $arr_users = $oUser->GetUsers();
 print_r($arr_users);