if ($stmt = $mysqli->prepare("SELECT District FROM City WHERE Name=?")) {

/* Lecture des marqueurs */
    $stmt->bind_param("s", $city);

<?php 
define('DSN', 'mysql:host=localhost;dbname=foetus69;port=3306');
define('LOGIN', 'root');
define('MOT_DE_PASSE', '');
$options = array();
?>

<?php
require_once('login.php'); // appel des paramètres de connexion
$dbh = new PDO(DSN, LOGIN, MOT_DE_PASSE, $options); // on se connecte

$var = 10;

$sql = 'SELECT id, nom, prenom from patients WHERE id = :idprepare'; //mon select avec mon marqueur nommé

$sth = $dbh->prepare($sql, array(PDO::ATTR_CURSOR => PDO::CURSOR_FWDONLY));// c'est quoi cette ligne ?
$sth->execute(array(':idprepare' => $var)); //on affecte et remplace la valeur

echo $sth->rowCount(); //je compte le nombre d'enregistrements à priori 1 ou 0

while($data=$sth->fetch()){ //je fais une affectation pour affichage
    echo "bonjour ".$data['prenom']." ".$data['nom'];
}
?>

PDO::ATTR_CURSOR (integer)
Selects the cursor type. PDO currently supports either PDO::CURSOR_FWDONLY and PDO::CURSOR_SCROLL. Stick with PDO::CURSOR_FWDONLY unless you know that you need a scrollable cursor.

PDO::CURSOR_FWDONLY (integer)
Create a PDOStatement object with a forward-only cursor. This is the default cursor choice, as it is the fastest and most common data access pattern in PHP.

PDO::CURSOR_SCROLL (integer)
Create a PDOStatement object with a scrollable cursor. Pass the PDO::FETCH_ORI_* constants to control the rows fetched from the result set.