like this

http://doc.silverstripe.com/doku.php?id=datamodel#overloading

“Getters” and “Setters” are functions that help us save fields to our data objects

but seems it does not work

<?php
class TestWidget extends Widget {
   static $db = array(
         "Myvalue" => "Text"
      );

   static $title = "Test widget";
   static $cmsTitle = "Test widget";
   static $description = "Test Test Test Test";

   function getCMSFields() {
      return new FieldSet(
         new TextField("Myvalue" , "Myvalue" )
      );
   }
   
   function getMyvalue(){
      if($this->Myvalue){
         return $this->Myvalue;
      }else{
         return "default value";
      }
   }
   
   function setMyvalue($value){
      $this->setField("Myvalue" , "hoho".$value);
   }
}
?>

i changed as following

function getMyvalue(){
      //return $this->getField("Myvalue");
      return "myvalue";
   }

it does not work even i return a string

and the
function setMyvalue($val){
$this->setField("Myvalue" , "prefix".$val."suffix");
}

it can not work too.
((