Intercepting filters are used to pre-process requests and post-process responses. They wrap and modify or transform inputs and outputs to/from core application logic. Intercepting Filter pattern can be applied at any level of an application. Limb uses intercepting filters as a part of FrontController. You can read more about this pattern at WACT DocuWiki - lmbInterceptingFilter

UML Static Structure:

}

UML Sequence:

}

It's easy to control core application logic with intercepting filters. Common Limb-based application is just a filter chain and all the application needs to do is to process this chain. Your index.php file can look like this:

require_once(dirname(__FILE__) . '/setup.php');
require_once(dirname(__FILE__) . '/src/MyApplication.class.php');
 
$application = new MyApplication();
$application->process(); 

And MyApplication class:

require_once(LPKG_CORE_DIR . '/src/filter/lmbFilterChain.class.php');
 
class MyApplication extends lmbFilterChain
{
  function __construct()
  {
    $this->registerFilter(new lmbHandle(LPKG_CORE_DIR . '/src/filter/lmbTimingFilter'));
    $this->registerFilter(new lmbHandle(LPKG_CORE_DIR . '/src/filter/ResponseProcessingFilter'));
    $this->registerFilter(new lmbHandle(LPKG_CORE_DIR . '/src/filter/lmbUncaughtExceptionHandlingFilter'));
    $this->registerFilter(new lmbHandle(LPKG_CORE_DIR . '/src/filter/lmbViewRenderingFilter'));
    $this->registerFilter(new lmbHandle(LPKG_CORE_DIR . '/src/filter/lmbRequestDispatchingFilter'));
    $this->registerFilter(new lmbHandle(LPKG_CORE_DIR . '/src/filter/lmbCommandProcessingFilter'));
  }
} 

You can think about Limb filter chain as of a set of nested filters, like this:

   +-authentication filter
   |
   | +-full page cache filter
   | | 
   | | +-other filter
   | | |
   | | |_
   | | 
   | |_
   |
   |_

It's up to the filter to decide whether to pass control to the next filter or not, e.g:

class SimpleInterceptingFilter implements lmbInterceptingFilter
{      
  function run($filter_chain)
  { 
    //pre-processing is done here
 
    if ($this->someConditionPassed())
      $filter_chain->next();    
 
    //post-processing is done here
  }      
}