Inscription php

salut a tous
j'ai une probleme au niveau de mysqli
je pas comprend cet erreur
( ! ) Fatal error: Call to a member function prepare() on a non-object in C:\wamp\www\PPP\verif.php on line 50
- ligen 50 : $stmt = $mysqli->prepare("INSERT INTO user VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)");
et voici mon code :
<?php
if(isset($_POST['b1']))
{
if (isset($_POST['n']))
{
$n=$_POST['n'];
}
if (isset($_POST['p']))
{
$n=$_POST['p'];
}

if (isset($_POST['psd']))
{
$n=$_POST['psd'];
}
if (isset($_POST['e']))
{
$n=$_POST['e'];
}
if (isset($_POST['pwd']))
{
$n=$_POST['pwd'];
}
if (isset($_POST['v']))
{
$n=$_POST['v'];
}
if (isset($_POST['a']))
{
$n=$_POST['a'];
}
if (isset($_POST['s']))
{
$n=$_POST['s'];
}
if (isset($_POST['sp']))
{
$n=$_POST['sp'];
}
$base= mysqli_connect("localhost", "root", "", "utilisateurs");

/* Vérification de la connexion */
if (mysqli_connect_errno()) {
printf("Échec de la connexion : %s\n", mysqli_connect_error());
exit();
}


$stmt = $mysqli->prepare("INSERT INTO user VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)");
$stmt->bind_param('sssd',$n,$p,$psd,$e,$pwd,$v,$a,$s,$sp);

$stmt->execute();

printf("%d ligne insérée.\n", $stmt->affected_rows);
$stmt->close();

$mysqli->close();
}

?>