I wish to produce a wordpress wordpress plugin that may catch Publish/GET demands to some certain URL from outdoors, such will be able to have exterior PHP code POSTing data towards the wordpress wordpress plugin, that the wordpress plugin may then further handle internally.

I'd rather not create blogs or pages from outdoors, I understand the XML-RPC interface is available for your. I am speaking about posting data from outdoors into wordpress after which handling it further inside a wordpress plugin.

I suppose I possibly could also directly connect with the Wordpress DB but that's most likely not really a very clean solution and can break basically upgrade Wordpress.

As you know about WordPress XMRPC: Your wordpress plugin can register own functions for XMLRPC that pay attention to the request then. You are able to send a Publish request towards the XMLRPC endpoint then and also the wordpress plugin function is going to be triggered then.

The next code-example is a straightforward wordpress plugin class that registers a function towards the XMLRPC endpoint my.recieve. It shows the way the callback function is registered to that particular string:

/**
 * XML RPC Plugin
 */
return RecievePostPlugin::bootstrap();

class RecievePostPlugin
{
    private static $instance;
    public static function bootstrap()
    {
        if (null === $this->instance)
             $this->instance = new __CLASS__;
        return $this->instance; 
    }
    public function __construct()
    {
        add_filter('xmlrpc_methods', array($this, 'addXMLRPCActions'));
    }
    public function addXMLRPCActions(array $methods)
    {
        $methods['my.recieve'] = array($this, 'recieveXMLRPCAction');
        return $methods;
    }
    public function recieveXMLRPCAction(array $arguments)
    {
       // your code here, POST can be accessed via $_POST
    }
}

Should you dislike the XMLRPC overhead, you can easily produce a PHP file a part of your wordpress plugin and ask for it. You will want to consider care you to ultimately load all Wordpress functions you might like to use (e.g. database access etc.).

You may create a rewrite rule, redirecting some whimsical pretty Hyperlink to personal files inside your plugin's directory. Which file may have $_Publish data. In the Wordpress page explaining Wordpress_Rewrite:

function feed_dir_rewrite( $wp_rewrite ) {
    $feed_rules = array(
        'index.rdf' => 'index.php?feed=rdf',
        'index.xml' => 'index.php?feed=rss2',
        '(.+).xml' => 'index.php?feed=' . $wp_rewrite->preg_index(1)
    );

    $wp_rewrite->rules = $feed_rules + $wp_rewrite->rules;
}

// Hook in.
add_filter( 'generate_rewrite_rules', 'feed_dir_rewrite' );