ERREUR Notice: Undefined variable: id... on line 5

<?php

require('model/model.php');

$post = getPost($id);

echo $post;

<?php

function getPost($id) {

    $db = dbConnect();

    try {
        $sql = 'SELECT id, title FROM general WHERE id = ?';
        $query = $db->prepare($sql);

        $query->execute(array($id));
        $return = $query->fetch();

        return $return;

    } catch(Exception $e) {
        echo 'Erreur dans la requête ' . $sql . " <br /> Erreur : ". $e->getMessage();
    }

    $idExist = !empty($return) ? true : false;
    if ($idExist) { return $return; } else { echo 'La variable n\'existe pas !'; }
}

function dbConnect() {

    try {
        $db = new PDO('mysql:host=localhost; dbname=web','root', 'root', array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
        return $db;
    }

    catch(Exception $e) {
        echo 'Erreur dans la requête ' . $sql . " <br /> Erreur : ". $e->getMessage();
    }
}