PHPのエラー『Notice: Undefined Property』の解決方法

class User {
    public $name;
}

$user = new User();
echo $user->age; // 未定義のプロパティにアクセス

Notice: Undefined property: User::$age in script.php on line X

class User {
    public $name;
    public $age; // プロパティを定義
}

$user = new User();
$user->age = 25;
echo $user->age; // 25

class User {
    public $name;
    public $age;

    public function __construct($name, $age) {
        $this->name = $name;
        $this->age = $age;
    }
}

$user = new User("Alice", 30);
echo $user->age; // 30

class User {
    public $name;
}

$user = new User();

if (isset($user->age)) {
    echo $user->age;
} else {
    echo "ageプロパティは定義されていません";
}

class User {
    private $data = [];

    public function __get($name) {
        return isset($this->data[$name]) ? $this->data[$name] : "プロパティ {$name} は存在しません";
    }

    public function __set($name, $value) {
        $this->data[$name] = $value;
    }
}

$user = new User();
echo $user->age; // プロパティ age は存在しません

class User {
    public int $age = 0;
}

$user = new User();
echo $user->age; // 0

$user = new stdClass();
$user->name = "Alice";
$user->age = 30;

echo $user->age; // 30