解决方法：

1.  原因是php版本太高8.1以上，可以通过降级完成。

2.  不降级的处理方案

You don't have to downgrade your PHP version unless you are using some third-party components also using jsonSerialize() and they are not able to fix it.

Here is what I did to fix it:

public function jsonSerialize():mixed

{

    // Some snippet here

    return $your_type;

}

Just give jsonSerialize() a return type, or mixed works just fine.