PHP有条件地扩展类？ （CodeIgniter 3使用案例）

I'm using the following scenario, but this doesn't necessarily pertain only to CodeIgniter.

HMVC: https://bitbucket.org/wiredesignz/codeigniter-modular-extensions-hmvc

Internationalization (i18n): https://github.com/waqleh/codeigniter-language-in-url-internationalization

Both HMVC & i18n work by extending CI_Config (and related CI_ classes).

To get i18n working alongside HMVC, I first have to require the appropriate HMVC file and then further extend that class.

So for example, CI_Config is extended by HMVC MX_Config and then further required & extended by i18n MY_Config.

But now i18n only works if HMVC is already in place.

What if HMVC is absent (no MX_Config and related)?

I want i18n to work both with/without HMVC.

Is there a way to conditionally require & extend one class or another? Or rather, extend CI_Config (and related) twice without collision? Or is there another way to accomplish what I want?

HMVC MX_Config partial:

/**
 * HMVC
 */
class MX_Config extends CI_Config
{

i18n MY_Config partial:

/* load HMVC's MX_Config class */
require APPPATH . "third_party/MX/Config.php";

/**
 * Language
 */
class MY_Config extends MX_Config
{

localheinz's answer is valid but does not take into account:

CodeIgniter loads core files CI_ & MY_ before HMVC MX_ . So there's no way to have MY_ check if an MX_ is declared.

Nor do I want to edit core CI_ files as they are not meant to be modified.

The working solution:

Note: The same logic below can be applied to other MY_ classes.

MY_Config partial:

/* load HMVC's MX_Config class */
if ( file_exists(APPPATH . "third_party/MX/Config.php") ) {
    require APPPATH . "third_party/MX/Config.php";
/* otherwise declare MX_Config class */
} else {
    class MX_Config extends CI_Config {}
}


/**
 * Language
 */
class MY_Config extends MX_Config
{

How about declaring the class MX_Config dynamically, if it doesn't exist?

<?php

class CI_Config {}

if (!class_exists('MX_Config')) {
    class MX_Config extends CI_Config {}
}

class MY_Config extends MX_Config {}

$config = new MY_Config();

var_dump(
    $config instanceof MX_Config,
    $config instanceof CI_Config
);

For an example, see: