在所有类中访问SQL连接变量？

I haven't touched PHP in a couple months and don't have access to any of my previous projects for reference. Essentially, I have my file lib.php which instantiates all objects, creates all vars/functions that will be accessed throughout most of the application. This file, lib.php is then called into the header.php file, viarequire_once. Within lib, I have a function which creates the MySQL PDO connection

try {
    $server = array(
            'server' => 'localhost:3306',
            'database' => 'to_do_list',
            'user' => 'root',
            'password' => '',
            );
    $link = new PDO('mysql:host='.$server['server'].';dbname='.$server['database'].'',$server['user'],$server['password']);
    $link->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
} catch (PDOException $e) {
    echo "Error Could not connect to database";
    exit;
}

I want the variables within here, such as $link to be accessible in all class files which are instantiated within lib.php. How would I go about doing this in the most efficient way?

<?php
// Session Start
session_start();

// Link Database
// Link var created here I want to be accessible in any classes instantiated below
try {
    $server = array(
            'server' => 'localhost:3306',
            'database' => 'to_do_list',
            'user' => 'root',
            'password' => '',
            );
    $link = new PDO('mysql:host='.$server['server'].';dbname='.$server['database'].'',$server['user'],$server['password']);
    $link->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
} catch (PDOException $e) {
    echo "Error Could not connect to database";
    exit;
}

// Functions
// Action GET/POST/SET
function getAction()
{
    if(isset($_POST['action']))
    {
        $action = $_POST['action'];
    } else if(isset($_GET['action'])) {
        $action = $_GET['action'];
    } else {
        echo "GET/SET/POST action master failed";
    }
    return $action;
}

// Input GET/POST/SET
function inputReturn($inputName) {
    if(isset($_POST[$inputName])) {
        $inputVal = $_POST[$inputName];
    } else if (isset($_GET[$inputName])) {
        $inputVal = $_GET[$inputName];
    } else {
        $inputVal = "";
        echo "GET/SET/POST input master failed";
    }
    return $inputVal;
}

// Require classes
require_once("user.php");

// Instantiate classes
$instUser = new user;

?>