[Passing Variables into auto loaded class files]

It seems that I'm struggling with an issue trying to pass a variable to the class file, but tells me the $db is undefined when I call the User::getUser(). Any ideas how can I achieve passing the $db that is defined in the bootstrap.inc.php and then using it in the auto loaded class file User?

 

 

In my bootstrap.inc.php file which it's included in the index.php file.

<?php
# Autoload the classes
spl_autoload_register("autoload");
# Autoload Composer Libraries
require_once('composer/autoload.php');
# Set up the database connection 
$db = \ParagonIE\EasyDB\Factory::create( 
	'mysql:host='.DB_HOST.';dbname='.DB_NAME, 
	DB_USERNAME, 
	DB_PASSWORD ); 

# Function to load the required classes files 
function autoload($class_name) 
{ 
	# Load classes... 
	if(file_exists(CLASS_PATH.DS.$class_name.'.class.php')) 
	{ 
		require CLASS_PATH.DS.$class_name.'.class.php';
		return; 
	} 
}

And then in my User.class.php file, I called the getUser function, and then it said undefined variable $db.

<?php
class User
{
    public static function getUser()
    {
        $userData = $db->row("SELECT * FROM users WHERE id = ?", 1);
        return $userData;
    }
}

I appreciate any solutions!