PHP ignoring if and giving ERR_TOO_MANY_REDIRECTS

Question

Hello, I develop a program for the store I work and when we order to open a coupon, it checks to see if it is opening an existing coupon (with the variable $_GET['cupom']) or if it is a new coupon. It worked very well, even existed a if with too much redirecting in this code before the last update, had never given error of Multiple Redirects but now it is giving I do not know why.

When the browser displays the error screen with the ERR_TOO_MANY_REDIRECTS, the displayed url is http://localhost/administrar/pedidos_venda.php?cupom=1&aberto=1 as if the else if(!$existeCodigoCupom) was executed. But if true, the program ignored the if's previous.

I learned everything I know by myself, through documentations and forums. It may have errors, feel free to indicate, so I learn a little more.

Follow the php code:

if(!isset($_GET['cupom']))
{

    $existeCodigoCupom = DBRead("vendapedidos", "ORDER BY id DESC LIMIT 1");
    $codigoCupom = DBRead("vendapedidos" , "WHERE status = '-1' AND aberto = '0' ORDER BY id ASC LIMIT 1");

    if($codigoCupom)
    {
        if($codigoCupom[0]['aberto'] == 0)
        {
            $codigo = $codigoCupom[0]['id'];

            $dados = array
            (
                'dataHoraCriacao' => date('Y-m-d H:i:s'),
                'dataHoraAtualizacao' => date('Y-m-d H:i:s'),
                'valores' => "0,,0",
                'status' => -1
            );

            $registraCupom = DBUpDate("vendapedidos", $dados, "id = '{$codigo}'");
            if($registraCupom)
            {
                header('Location: /administrar/pedidos_venda.php?cupom='.$codigo);
                exit();
            }
            else
            {
                echo "Não foi possível abrir o cupom!";
                exit();
            }
        }
    }
    else if($existeCodigoCupom)
    {
        $codigo = $codigoCupom[0]['id'];
        $codigo++;

        $dados = array
        (
            'dataHoraCriacao' => date('Y-m-d H:i:s'),
            'dataHoraAtualizacao' => date('Y-m-d H:i:s'),
            'valores' => "0,,0",
            'status' => -1
        );

        $registraCupom = DBCreate("vendapedidos", $dados);
        if($registraCupom)
        {
            header('Location: /administrar/pedidos_venda.php?cupom='.$codigo);
            exit();
        }
        else
        {
            echo "Não foi possível abrir o cupom!";
            exit();
        }
    }
    else if(!$existeCodigoCupom)
    {
        $codigo = 1;

        $dados = array
        (
            'dataHoraCriacao' => date('Y-m-d H:i:s'),
            'dataHoraAtualizacao' => date('Y-m-d H:i:s'),
            'valores' => "0,,0",
            'status' => -1
        );

        $registraCupom = DBCreate("vendapedidos", $dados);
        if($registraCupom)
        {
            header('Location: /administrar/pedidos_venda.php?cupom='.$codigo);
            exit();
        }
        else
        {
            echo "Não foi possível abrir o cupom!";
            exit();
        }
    }
}