PHP Autoload – How to Load Two Class Path Directories with __autoload()

How should I design a PHP class autoloader?

You shouldn't.

Leads from a number of PHP frameworks (Zend Framework, CakePHP, etc) and applications (Joomla, phpBB, etc) came together in late 2009 to build the PSR-0 Autoloader Proposal, which dictates a class file naming scheme and namespace mechanism.

It's remarkably similar to your existing system, with the simple inclusion of PHP 5.3 namespaces.

It's simple, it's straightforward, and if all goes well, it'll Just Work(tm) with a large body of third party code in the future.

It looks like you are having a design issue here: Tables should not extend the Database Abstraction Layer. Instead the DAL should be injected into the table as a dependency.

abstract class Table {
    protected $dal;

    public function __construct(DAL $dal) {
        $this->dal = $dal;
    }

    // whatever else all tables have in common
}

class Table_User extends Table {
    public function someMethod() {
        $this->dal->someOtherMethod();
    }
}

$table = new MyTable($dal);
$table->someMethod();

That way you will create a DAL at some upper scope and pass it down. This will also allow you to use multiple different database engines at the same time.

Additionally you obviously should not create your tables directly in your controller but let a specialized class do that. For example you could use a factory:

class TableFactory {
    protected $dal;

    public function __construct(DAL $dal) {
        $this->dal = $dal;
    }

    public function createTable($name) {
        $className = 'Table_' . $name;
        return new $className($this->dal);
    }
}

That way you can create a table factory at some point with an injected DAL and pass that table factory around.

$table = $factory->createTable('User');