SystemActionEnableShortcodeHelloWorld.class.php

<?php

namespace myNamespace;

//To use this class add this line to your plugin file:
// add_shortcode('helloWorld', array(new $SystemActionEnableShortcodeHelloWorld, 'returnShortcode'));

class SystemActionEnableShortcodeHelloWorld{

	public function returnShortcode(){
		$stringHellowWorld = __('Hello world!', 'textDomain');

$shortCodeOutput = <<<stringHellowWorldHerdoc
$stringHellowWorld
shortCodeOutputHerdoc;

		return $shortCodeOutput;
	}
}