// Creates instance of Zend_Markup_Renderer_Html,
// with Zend_Markup_Parser_BbCode as its parser
$bbcode = Zend_Markup::factory('Bbcode');

// this will create a simple 'foo' markup
// The first parameter defines the markup's name.
// The second parameter takes an integer that defines the markups type.
// The third parameter is an array that defines other things about a
// markup, like the markup's group, and (in this case) a start and end markup.
$bbcode->addMarkup(
    'foo',
    Zend_Markup_Renderer_RendererAbstract::TYPE_REPLACE,
    array(
        'start' => '-bar-',
        'end'   => '-baz-',
        'group' => 'inline'
    )
);

// now, this will output: 'my -bar-markup-baz-'
echo $bbcode->render('my [foo]markup[/foo]');

By adding a callback markup, you can do a lot more then just a simple replace of the markups. For instance, you can change the contents, use the parameters to influence the output etc.

A callback is a class that implements the Zend_Markup_Renderer_TokenInterface interface. An example of a callback class:

class My_Markup_Renderer_Html_Upper
    implements Zend_Markup_Renderer_TokenConverterInterface
{

    public function convert(Zend_Markup_Token $token, $text)
    {
        return '!up!' . strtoupper($text) . '!up!';
    }

}

Now you can add the 'upper' markup, with as callback, an instance of the My_Markup_Renderer_Html_Upper class. A simple example:

// Creates instance of Zend_Markup_Renderer_Html,
// with Zend_Markup_Parser_BbCode as its parser
$bbcode = Zend_Markup::factory('Bbcode');

// this will create a simple 'foo' markup
// The first parameter defines the markup's name.
// The second parameter takes an integer that defines the markups type.
// The third parameter is an array that defines other things about a
// markup, like the markup's group, and (in this case) a start and end markup.
$bbcode->addMarkup(
    'upper',
    Zend_Markup_Renderer_RendererAbstract::TYPE_CALLBACK,
    array(
        'callback' => new My_Markup_Renderer_Html_Upper(),
        'group'    => 'inline'
    )
);

// now, this will output: 'my !up!MARKUP!up!'
echo $bbcode->render('my [upper]markup[/upper]');