Problema com PDO

class UsuarioAcessaDAO implements DAO{
	
	private $conex;
	
	public function UsuarioAcessaDAO(){
		$this->conex = new Conexao('mysql');
	}
	
	public function selectRelatorios($usuarioAcessa){
		try{
			$stmt = $this->conex->prepare("
					select
						ua.id_usuario as usuario,
						ua.id_relatorios as relatorio
					from
						rel_usuario_acessa ua
					where
						ua.id_usuario = ?");
			
			$stmt->bindValue(1, $usuarioAcessa->getUsuario());
			
			$this->conex = null;
			return $stmt;
		}catch ( PDOException $ex ){
			echo "Erro: ".$ex->getMessage();
		}
	}

	public function selectModulos(){
		try{
			
			$sql = 'select distinct m.id_modulo, m.sigla from rel_usuario_acessa ua 
					left join rel_relatorios r on ua.id_relatorios = r.id_relatorios
					left join rel_modulos m on r.id_modulo = m.id_modulo where ua.id_usuario = ';
			
			
			$stmt = $this->conex->prepare($sql);
			
			$this->conex = null;
			
			$i=0;
			while($row = $stmt->fetch(PDO::FETCH_ASSOC)){
				$modulos[$i]['ID'] = $row['ID_MODULO'];
				$modulos[$i]['SIGLA'] = $row['SIGLA'];
			}
			
			return $modulos;
			
		}catch ( PDOException $ex ){
			echo "Erro: ".$ex->getMessage();
		}
	}
}