Implement message board function:
1. The write.php file implements writing characters into local text:
```php
```
2. The index.php file implements the front page for inputting content and displays the content that has been left:
```php
```
`$string = rtrim($string, '&^');` in the index.php file. I don't understand this code. "&^" is the demarcation of a message. The character "&^" has been removed here. Then the next sentence` $arr = explode('&^', $string);` How can we still use the character "&^"?
