sampleFilter.php

<?php

class MyClass extends Page{
  
  public static function getFilteredList(SS_HTTPRequest $request = null){
    //assume the request var holds a valid SS_HTTPRequest
    
    $list = MyObject::get();
    
    if($someVar = $request->getVar('Somevar')){
      $list = $list->filter('Somevar', $someVar);
    }
    
    if($anotherVar = $request->getVar('AnotherVar')){
      $list = $list->filter('AnotherVar', $anotherVar);
    }
    
    //to this point the list is filtered fine with proper results
    
    if($yetAnother = $request->getVar('YetAnother')){
      $filterByThat = function($obj){
        return $obj->myFunctionThatCalcs();//function returns true/false on the MyObject being iterated over
      };
      
      $list = $list->filterByCallback($filterByThat);
    }
    
    //list now only filtered by the callback if YetAnother is set as a GET var
    return $list;
    
  }
  
  
}