Error inserting data into MySQL - CodeIgniter

The problem is basically in the query, it seems that it is trying to insert the value of a string, like number, and gives error, try casting a type:

<?php
$chave = $retorno->resposta->cobrancasGeradas->cliente->cobranca->chave;

$this->db->insert('boletos', array('id_fatura' => $id_fatura, 'chave_boleto' => (string) $chave));
?>

or this:

 $this->db->insert('boletos', array('id_fatura' => $id_fatura, 'chave_boleto' => "{$chave}"));

You can do it this way:

<?php
     $chave = $retorno->resposta->cobrancasGeradas->cliente->cobranca->chave;
     $dados['id_fatura'] = $id_fatura;
     $dados['chave_boleto'] = $chave;
     $this->db->insert('boletos', $dados);
?>

Just adapt to your mood.

Correct way with PDO. The good thing is you do an abstract class with the CRUD (abstract class Crud extends DB) put this code and replace the values with variables, then just call the crud in your class you want to do the insert.

But not thick is there.

    try {
            DB::getInstance()->beginTransaction();

            $sql = "INSERT INTO boletos (id_fatura,chave_boleto) VALUES (:id_fatura, :chave_boleto)";
            $stmt = DB::prepare($sql);
            $dados = seuArray //array('id_fatura'=>$id_fatura, 'chave_boleto'=>$chave)

            foreach ($dados as $key => $v) {
                $stmt->bindValue(':' . $key, $v);
            }
            //ou assim
            //$stmt->bindValue(':id_fatura', 1);
            //$stmt->bindValue(':chave_boleto', 'fjçalkdfjoajlaksdf');


            if ($stmt->execute()) {
                $cd = DB::getInstance()->lastInsertId();
                modalSalvo("Salvo com sucesso!");
                return $cd;
                DB::getInstance()->commit();
            }
        } catch (PDOException $e) {
            echo '<br />Error: ' . $e->getMessage();
        }