test.php

class ParentTest{
	private $log;

	public function __construct(){
		$this->log=fopen('test.log','a+');
		add_action('send_headers',array($this,'testA'));
		add_filter('the_content',array($this,'testB'));
		add_action('send_headers',array($this,'testC'));
		add_filter('the_content',array($this,'testD'));
		add_filter('the_content',array($this,'showAll'));
	}

	public function testA(){
		$log_message="---Test A (".get_class($this)." - hook:send_headers)---\n\n".
		get_query_var('test','Can\'t retrieve test var')."\n".
		get_query_var('error','Can\'t retrieve error var')."\n\n";

		fwrite($this->log,$log_message);
	}

	public function testB($contents){
		$log_message="----Test B---(".get_class($this)." - hook:the_content)\n\n".
		get_query_var('test','Can\'t retrieve test var')."\n".
		get_query_var('error','Can\'t retrieve error var')."\n\n";

		fwrite($this->log,$log_message);
	}

	public function testC(){
		$test=(isset($_GET['test']))? $_GET['test']: 'Can\'t retrieve test var';
		$error=(isset($_GET['error']))? $_GET['error']: 'Can\'t retrieve error var';

		$log_message="---Test C (".get_class($this)." - hook:send_header) ---\n\n".$test."\n".$error."\n\n";

		fwrite($this->log,$log_message);
	}

	public function testD(){
		$test=(isset($_GET['test']))? $_GET['test']: 'Can\'t retrieve test var';
		$error=(isset($_GET['error']))? $_GET['error']: 'Can\'t retrieve error var';

		$log_message="---Test D (".get_class($this)." - hook:the_content)---\n\n".$test."\n".$error."\n\n";

		fwrite($this->log,$log_message);
	}

	public function showAll($contents){
		global $wp_query;
		$log_message="--- Show All (- hook:the_content)---\n\n".print_r($wp_query->query_vars,1)."\n\n";
		fwrite($this->log,$log_message);

	}

	public function __destruct(){
		fclose($this->log);
	}
}

class TestClassA extends ParentTest{
	public function __construct(){
		add_action('init',function() { 
		    global $wp; 
		    $wp->add_query_var('test');
		    $wp->add_query_var('error'); 
		});

		parent::__construct();
	}
}

class TestClassB extends ParentTest{
	public function __construct(){
		add_filter('query_vars',function($vars){
  			$vars[] = 'test';
  			$vars[] = 'error';
	  		return $vars;
		});

		parent::__construct();
	}
}

$a=new TestClassA;
$b=new TestClassB;