Joomla! Documentation™

A common example of using plugins is to run the content plugins on some text. This is useful if you want to support plugins that usually work on Content from a custom extension. For the content prepare trigger you can simply call:

$text = JHtml::_('content.prepare', $text);

For any other content triggers you must call:

$dispatcher = JDispatcher::getInstance();
$item->text = your_text_area_item;  
$item->params = clone($params);
JPluginHelper::importPlugin('content'); 
$dispatcher->trigger('onPrepareContent', array (& $item, & $item->params, 0));

You might want to look at core components (for example com_content) for an example. See the triggers page for the possible content plugin triggers.

Also for PHP5.3 compliance please look at the discussion page.