PHP - duda sobre php

<?php

session_start();

include 'primeraparte.php';

include 'segundaparte.php';

include 'terceraparte.php';

echo "prueba funcionamiento";

session_destroy();

?>

if (isset($_POST['tarjeta'])) {

       include 'redsys.php';

 }

session_start();

  $amount = $_SESSION['totalcomp'];

    include "tpv/apiRedsys.php";

    $miObj = new RedsysAPI;

    $url_tpv = 'https://sis-t.redsys.es:25443/sis/realizarPago';

    $version = "HMAC_SHA256_V1";

    $clave = 'XXXXXXXXXXXXXXXXXXXXXXXXX'; //poner la clave SHA-256 facilitada por el banco 

    $name = 'INFO SUPUESTO CLIENTE';

    $code = '111111';

    $terminal = '0';

    $order = date('ymdHis');

    $amount = $amount * 100;

    $currency = '874';

    $consumerlng = '455';

    $transactionType = '8';

    $urlMerchant = 'http://MIPAGINA.es/';

    $urlweb_ok = 'http://MIPAGINA.es/conexion.php';

    $urlweb_ko = 'http://MIPAGINA.es/fail.php';

    $miObj->setParameter("DS_MERCHANT_AMOUNT", $amount);

    $miObj->setParameter("DS_MERCHANT_CURRENCY", $currency);

    $miObj->setParameter("DS_MERCHANT_ORDER", $order);

    $miObj->setParameter("DS_MERCHANT_MERCHANTCODE", $code);

    $miObj->setParameter("DS_MERCHANT_TERMINAL", $terminal);

    $miObj->setParameter("DS_MERCHANT_TRANSACTIONTYPE", $transactionType);

    $miObj->setParameter("DS_MERCHANT_MERCHANTURL", $urlMerchant);

    $miObj->setParameter("DS_MERCHANT_URLOK", $urlweb_ok);

    $miObj->setParameter("DS_MERCHANT_URLKO", $urlweb_ko);

    $miObj->setParameter("DS_MERCHANT_MERCHANTNAME", $name);

    $miObj->setParameter("DS_MERCHANT_CONSUMERLANGUAGE", $consumerlng);

    $params = $miObj->createMerchantParameters();

    $signature = $miObj->createMerchantSignature($clave);

    ?>

    <form id="realizarPago" action="<?php echo $url_tpv; ?>" method="post">

        <input type='hidden' name='Ds_SignatureVersion' value='<?php echo $version; ?>'>

        <input type='hidden' name='Ds_MerchantParameters' value='<?php echo $params; ?>'>

        <input type='hidden' name='Ds_Signature' value='<?php echo $signature; ?>'>

    </form>

    <p>Un momento por favor...</p>

    <script>

    $(document).ready(function () {

        $("#realizarPago").submit();

    });

    </script>

<?php

session_start();

if (!isset($_SESSION["carrito"])) {

header('Location: http://www.mipagina.com/index.php');

     }else{

include 'includes/cliente.php';

header('Location: http://www.mipagina.com/compra.php');

    }

 ?>

<?php

session_start();

$conexion = new mysqli('datos conexion','datos conexion', 'datos conexion', 'datos de conexion');

if ($conexion->connect_errno) {

  die('lo siento hubo un problema con el servidor');

} else {

$total=0;

 if(!empty($_SESSION["carrito"])){

     $nombre =  $_SESSION['nombres'];

       $email =  $_SESSION['emails'];

       $telefono = $_SESSION['telefonos'];

       $fechapedido=$_SESSION['fechas'];

       $total=$_SESSION['totalcomp'];

       $fecha_hora_actual = date('Y-m-d H:i:s');

$prueba = array($nombres,$emails,$telefonos,$fechapedidos,$fecha_hora_actual);

$statement = $conexion-> prepare("INSERT INTO cliente(id,nombres,emails,telefonos,fechas,fechapedido) VALUES (?,?,?,?,?,?)");

$statement->bind_param('ississ',$id,$nombre,$email,$telefono,$fechapedido,$fecha_hora_actual);

$statement->execute();

 }

}

 ?>

if( !$statement ->execute() ) echo $statement ->error;