pdo_connect.php
=================

class PDOContainer
{
    public static $pdo = NULL;

    public function __construct()
    {
        if (!self::$pdo)
        {
            PDOContainer::$pdo = new PDO(
                'mysql:host='.DB_HOST.';dbname='.DATABASE,
                DB_USER, DB_PASSWORD,
                Array(PDO::ATTR_PERSISTENT => TRUE));
        }
    }
}

 other_script.php
==================
require_once('pdo_connect.php');
class SomeClass extends PDOContainer
{
    public function doSomething()
    {
        self::$pdo->prepare('SELECT * FROM my_table'); // they all share the same $pdo anyway
    }
}

The moment an object of class SomeClass is created, the PDOContainer::$pdo will be initialized