问PHP致命错误:在第9行C:/xampp/htdocs/oops/1.php中未在对象上下文中使用$this？

EN

<?php

class One {

    public static $a = 10;
    public static $b = 20;

    public static function onec() {
        self::$a = self::$b;
        return self::$a;
    }

}
echo One::onec();

<?php

class One {

    public $a = 10;
    public $b = 20;

    public static function onec() {
        $obj = new One();
        $obj->a = $obj->b;
        return $obj->a;
    }

}

echo One::onec();

class One {
   public $a = 10;
   public $b = 20;

   public function onec() {
     $this->a = $this->b;
     return $this->a;
   }
}
$obj = new One();
echo $obj->onec();