Including parent objects in "require"d files - php

Question

Overview
I connect to a database in index.php, then import my classes.php file using require_once(). However, when connecting to the database, the database connection is undefined.
Code
index.php
$dbh = new PDO("mysql:host=$hostname;dbname=$dbname",$username,$password);
require_once("inc/classes.php");
/* ..... */
if($_POST["form"]=="login"){
//Retrieve values
$e = $_POST["email"];
$p = $_POST["password"];
//Data validation
if(!filter_var($e, FILTER_VALIDATE_EMAIL)||strlen($e)<3) $errors->addError("email", "Please enter a valid email address.");
if(strlen($p)<1) $errors->addError("password", "Please enter a valid password");
$errors->killErrors();
//Log user in
$user = new User($e);
if(!$user->login($p)) $errors->addError("form", "Incorrect username or password.");
$errors->killErrors();
exit("success");
}
inc/classes.php
class User
{
public $id, $email, $data;
public function __construct($e = null){
if(isLoggedIn()){
$stmt = $dbh->prepare("SELECT * FROM `users` WHERE `id`=? LIMIT 1");
$stmt->execute(array($_SESSION["userid"]));
$this->data = $stmt->fetch(PDO::FETCH_ASSOC);
} else $this->email = $e;
}
public function login($p){
//Perform database query for user
$stmt = $dbh->prepare("SELECT `id`, `password` FROM `users` WHERE `email`=? LIMIT 1");
$stmt->execute(array($this->email));
if($stmt->rowCount()<1) return false;
//Check password is correct
$data = $stmt->fetch(PDO::FETCH_ASSOC);
if(!password_verify($p, $data["password"])) return false;
if(!$this->email) exit("User can only be logged in with an email address, not by User ID");
$this->id = $data["id"];
return $this->validate($this->id) ? true : false;
}
}
Output
Notice: Undefined variable: dbh in /var/www/html/foo/public_html/bar/inc/classes.php on line 80
Fatal error: Call to a member function prepare() on null in /var/www/html/foo/public_html/bar/inc/classes.php on line 80
Line 80 is:
$stmt = $dbh->prepare("SELECT `id`, `password` FROM `users` WHERE `email`=? LIMIT 1");
Question
How can I include the database connection in index.php and have the classes.php file accept that PDO object?

Answer 1

You have to make your connection global:
global $dbh;
Because of scope. Done carefully it is OK to use global variables. Many use it too haphazardly and have a tendency to want to globalize all of their variables. I personally do it within the function inside the class to eliminate ambiguity.

Answer 2

http://php.net/manual/en/language.variables.scope.php - as #JayBlanchard writes, you need to reference the connection handle as a global variable.
However, it's much cleaner to pass dependencies into your object. For instance, when instantiating your User, you can pass in the connection.
$user = new User($dbh, $e);
You can then set a private variable for the connection handle. This way, you can change the name of the variable in index.php, or change the way it's instantiated without your User class blowing up due to a change in a different file.