Bu işlevi hatayı düzeltmek için nasıl

 class MyClass {
  private $_callingscript; 
             public   function __construct(){
    $this->_callingscript= basename($_SERVER['SCRIPT_NAME']);
        }
        public static function Setvalue($k,$v){
   if (!empty($k)) {
   $_SESSION[$this->_callingscript.'_'.$k]= $v;//This doesnot work 
   $_SESSION[$_SERVER['SCRIPT_NAME'].'_'.$k]=$v //This works 

  }
        }
 }

 MyClass::setValue('Somename',"Somevalue");

When i call this it is giving me error "Using $this when not in object context in". How to correct the callingscript variable .Is this due to private declaration of that variable