function my_plugin_parse_request($wp) {
	// only process requests with "my-plugin=ajax-handler"
	if (array_key_exists('my-plugin', $wp->query_vars) && $wp->query_vars['my-plugin'] == 'ajax-handler') {
		header('Content-Type: text/plain');
		global $wp;
		
		if(isset($wp)){
			echo "got wp";
//			print_r($wp);

			$cats = get_categories('orderby=count&order=desc');
			
			print_r($cats);
		}
		exit;
	}
}
add_action('parse_request', 'my_plugin_parse_request');

function my_plugin_query_vars($vars) {
	$vars[] = 'my-plugin';
	return $vars;
}
add_filter('query_vars', 'my_plugin_query_vars');