Error in JSON with Accented Words

Question

I have a code in PHP, which prints values from the database in json.

<?php
    header('Content-Type: application/json; charset=utf-8');

    require_once "conexao.php";

    class Modelo {

      private $pdo;

      function Modelo() {
        $conexao = new Conexao();
        $this->pdo = $conexao->conect();
      }

      public function listaClientes() {

        $sql = "SELECT * from cliente";
        $sql = $this->pdo->query($sql);

        $lc = array();

        foreach($sql->fetchAll() as $cliente) {
          $lc[] = $cliente['nome'];
        }

        return json_encode($lc);
      }

    }

    $m = new Modelo();

    echo $m->listaClientes();

    echo json_last_error();

    ?>

It runs normal, but if I set some value with accent it shows nothing, just returns the value 5 of the function json_last_error, which in this case is an error with respect to UTF8

• 5 = JSON_ERROR_UTF8: UTF-8 encoding error of JSON string;

My bank already has the correct coding, how can I solve this problem?