Insert data with PHP to BD Mysql

I have started learning PHP relatively recently and in the process I am trying to insert the data of a form into a database using PHP as I have been looking at a safe way by means of ready statements and objects and so I am trying it even though I am not if it is the correct form I enclose the data insertion code.

<?php
include('conexion.php');
if (isset($_POST['submit'])) {

$codLibro=$_POST['codLibro'];
$importe=$_POST['importe'];
$tipoMoneda=$_POST['tipoMoneda'];

$peticion="INSERT INTO ejemplares (codLibro,importe,tipoMoneda) 
VALUES (?, ?, ?)";

$stmt = $conn->prepare($peticion);
$stmt->bind_param('dds', $codLibro, $importe, $tipoMoneda);
$stmt->execute();
$stmt->close();

echo "Registro Insertado";
$conn->close();
}
?>

Here I put my PHP file of connection to the Database

Below is my form for sending data in HTML

<form action="addejemplares.php" method="POST">
    <div class="form-group">
        <div class="col-xs-2">

            <label for="codLibro">codLibro:</label>
            <input type="text" class="form-control" name="codLibro" id="codLibro">
            <label for="importe">importe:</label>
            <input type="text" class="form-control" name="importe" id="importe">
            <label for="tipoMoneda">tipoMoneda:</label>
            <input type="text" class="form-control" name="tipoMoneda" id="tipoMoneda">
            <button type="submit" class="btn btn-default" name="submit">Submit</button>
        </div>
        </div>
</form>

As you can see, a simple submission form and then I attached an image of the table to which I want to insert data

And finally I enclose an image of the errors that PHP is taking me First of all thank you very much in advance to the entire community.

Connection

<?php
$conn = new mysqli("localhost", "root", "", "bdBiblioteca");

if (mysqli_connect_errno()) {
die("No se puede conectar a la base de datos:" . mysqli_connect_error());
}else{

    echo "conexion exitosa";
}

$conn->close();

?>