This function doesn't appear to work at all. You have to create a new BCMath\Number object to use it, at which point it attempts to overwrite the value of said object - and of course that won't work because BCMath\Number::$value is readonly.
(PHP 8 >= 8.4.0)
BcMath\Number::__unserialize — data 引数を BcMath\Number オブジェクトとして復元する
dataこのメソッドは、不正なシリアライズデータが渡された場合、ValueError をスローします。
This function doesn't appear to work at all. You have to create a new BCMath\Number object to use it, at which point it attempts to overwrite the value of said object - and of course that won't work because BCMath\Number::$value is readonly.