PHP对象...为什么在类中实例化类是其他函数不可见的属性？

Question

I've looked around a lot for help on this. I realise this is probably more to do with the way I am using objects (I'm new to OO PHP) but it's really bugging me. Here is a massively simplified version of what I'm trying to do:

<?php
class Show_message {
    public $message_instance = ""; //ensure Message object variable is visible
    function __construct() {
    //do nothing
    }
    function display_message() {
        $message_instance = new Message(); //instatiate Message object
        echo $message_instance->message . " : in display_message function <br>"; //works
    }
    function display_again() {
        echo $message_instance->message . " : in display_again function <br>"; //does not work
    }
}
class Message {
    public $message = ""; //ensure $this->message variable is visible?
    function __construct() {
        $this->message = "Hello world"; //make message
    }
}
$instance = new Show_message(); //instatiate Show_message object
$instance->display_message(); //method to create instance and display message
$instance->display_again(); //method to display message again
?>

Why is the $message_instance->message not visible to the display_again() function?

Answer 1

You need to use $this->, otherwise you are storing the Message object locally to the function, not to the class instance.

$this->message_instance = new Message();

and $this->message_instance everywhere you are doing $message_instance.