类中未定义的变量…但是我定义了它

Question

I'm trying to create a class for my database: class structure as below

<?php
    require_once("config.php"); // include(Username, passowrd, server and databaseName)
    class MySQLDatabase{
        private $connection;

        function __construct(){
            $this->openConnection();
        }

        public function openConnection(){
            $this->$connection = mysqli_connect(DB_SERVER, DB_USER, DB_PWD, DB_NAME);
            if(!$connection){
                die("Database connection failed: ".mysql_error);
            }
        }

    }
    $database = new MySQLDatabase();
    $db =&$database;

 ?>

then my first step was check this weather work or not. so I code index.php file as below

    <?php                   
require("../includes/database.php");                                       if(isset($database)){
echo "true";
}else{
echo "false";
}
    ?>

after that I run the program and I got errors as below. I can not understand the reason. I google it and there are no clear solution for this. is there are anyone can help me to understand why this happen and how to solve this?

( ! ) Notice: Undefined variable: connection in C:\\wamp\\www\\waytoits\\includes\\database.php

on line 9
Call Stack
#   Time    Memory  Function    Location
1   0.0005  240792  {main}( )   ..\index.php:0
2   0.0008  249872  require( 'C:\wamp\www\waytoits\includes\database.php' ) ..\index.php:3
3   0.0013  250528  MySQLDatabase->__construct( )   ..\database.php:55
4   0.0013  250592  MySQLDatabase->openConnection( )    ..\database.php:15

( ! ) Fatal error: Cannot access empty property in C:\wamp\www\waytoits\includes\database.php on line 9
Call Stack
#   Time    Memory  Function    Location
1   0.0005  240792  {main}( )   ..\index.php:0
2   0.0008  249872  require( 'C:\wamp\www\waytoits\includes\database.php' ) ..\index.php:3
3   0.0013  250528  MySQLDatabase->__construct( )   ..\database.php:55
4   0.0013  250592  MySQLDatabase->openConnection( )    ..\database.php:15

Answer 1

You are using $this->$connection instead of $this->connection . Remove the $ sign in connection and try again.

Answer 2

$this::$connection (with the $ preserved) would refer to a static variable. Because you have a class variable you should access it via $this->connection

Also you need to use the same instructions in your if -statement.