Unset semantic on instance var

Hi, try: _______ class A { protected $a; public function __construct() {

       if(!isset($this->a))
       {
           $this->a = 'd';

echo '1st';

       }
       unset($this->a);
       if(!isset($this->a))
       {
           $this->a = 'g';

echo '2nd';

       }

}

    public function __set($columnName, $value)
    {
        echo 'New value : ' . $value . ' for '. $columnName;
    }

} new A(); ______ unset mean removing the instance variable from the class!! I was using the lazy initialisation with the isset function but then now I will initalize all inst var with null and check with the is_null function. Would it be possible to have inst var null by default insthead of a pseudo unset/null variable. Thanks Cheers - Mathieu Suen