What is wrong with this GTKSourceView syntax highlighting code?

I'm runnig this simple PHP-GTK2 code to highlight PHP source code,
based on this article:
http://www.php-gtk.eu/code-snippets/extending-gtksourceview-widget

The problem is that the GTKSourceView widget stops doing correct
highlight if you type some invalid PHP code then correct it.
I installed PHPGTk2 in WIndows XP using this Gnope installation:
http://www.gnope.org/download.php

Why does it stops hightlting  syntax?
The code follows:


<?php
error_reporting(E_ALL);

class EditWidget extends GtkSourceView {
	protected $buffer;
	protected $lang;
	protected $mime_lang;

	function __construct()
	{
		parent::__construct();

		# default lang
		$this->mime_lang = 'text/x-php';
		$this->set_lang($this->mime_lang);

		$this->buffer = new GtkSourceBuffer();
		$this->set_buffer($this->buffer);

		$this->set_show_line_numbers(true);
		$this->set_show_line_markers(true);

		$this->buffer->set_language($this->lang);
		$this->buffer->set_highlight(true);
	}

	protected function set_lang($mime_type) {
		$lang_manager = new GtkSourceLanguagesManager();
		$this->lang = $lang_manager->get_language_from_mime_type($mime_type);
	}

	public function load_file($file) {
		if (!file_exists($file))
			return false;

		$lines = file_get_contents($file);

		$this->buffer->begin_not_undoable_action();
		$this->buffer->set_text($lines);
		$this->buffer->end_not_undoable_action();
		return true;
	}
}






// Create our windows, centered, with scrollbars and the EditWidget
class App extends GtkWindow
{
	protected $scrolled_win;
	protected $edit_widget;

    function __construct()
    {
        parent::__construct();

		$edit_widget = new EditWidget();
		$edit_widget->load_file(__FILE__);
    	$scrolled_win = new GtkScrolledWindow();
		$scrolled_win->set_policy(Gtk::POLICY_AUTOMATIC, Gtk::POLICY_AUTOMATIC);
		$scrolled_win->add_with_viewport($edit_widget);
		parent::add($scrolled_win);

		parent::set_title("GtkSourceView Syntax Highlight");
        parent::set_size_request(600,480);
		parent::set_position(Gtk::WIN_POS_CENTER);
        parent::connect_simple('destroy', array("Gtk", 'main_quit'));
        parent::show_all();
    }
}

new App;
Gtk::Main();

?>