GetOptionKit\Option::filter PHP Method

    public function filter($cb)
    {
        $this->filter = $cb;
        return $this;
    }

 public function testFilter()
 {
     $opt = new Option('scope');
     $opt->filter(function ($val) {
         return preg_replace('#a#', 'x', $val);
     });
     $opt->setValue('aa');
     is('xx', $opt->value);
 }