Truncated incorrect DOUBLE value. What error is that?

On this page I select the book id to display it and then refresh the visits field by incrementing it.

<?php 

if(isset($_GET['id']))
{
    $id = $_GET['id'];
}
$livros = mysqli_query($conexao, "SELECT nome, descricao, imagem, download, 
visitas FROM livro WHERE id='$id'")or die(mysqli_error($conexao));
if(mysqli_num_rows($livros) <= 0)
{
    echo "<h2>Banco de dados não retornou nenhum valor</h2>";
}
while($res_livros = mysqli_fetch_array($livros))
{
    $nome = $res_livros['0'];
    $descricao = $res_livros['1'];
    $imagem = $res_livros['2'];
    $download = $res_livros['3'];
    $visitas = $res_livros['4'];
}
?>
<h1><?=$nome;?></h1>
<a data-fancybox="gallery" href="uploads/livros/<?=$imagem;?>">
    <figure>
    <img class="alignright" width="200" src="uploads/livros/<?=$imagem;?>"/>
      <figcaption><a id="download" href="<?=$download?>">Download</a> 
      </figcaption>
   </figure>
</a>
<h3><?=$descricao?></h3>
</section>
</div>

<?php

if(isset($visitas))
{
    $add_visitas = $visitas + 1;
    $up_visitas = mysqli_query($conexao, "UPDATE livro SET visitas = 
    '$add_visitas' WHERE id='$id'")or die(mysqli_error($conexao));
}

?>

To post the whole code but the error happens is down here in the UPDATE book.

Now, I decided to print the id here below and the id of the book + ". php" appeared like this: 106.php. That's why Boolean is giving, but where does this ".php" come from? In the database is just the normal number.

I solved the problem here, it was bullshit of me, obviously I passed the .php along with the id to the page but for some reason SELECT had no conflict with that, but UPDATE did.