尝试启动连接时为什么会出现NULL？

Question

I have a class with global $connection but when I try to access it I am getting NULL. As you can see if i try to access within the constructor I do not get NULL. But from getConnection() I am getting NULL.

class DatabaseManipulation
{
    private $connection;

    function __construct()
    {
        global $connection;
        $connection = new mysqli("localhost", "root", "", "db");
        $result = $connection->query("select * from user");

        print_r($result); //I get an array
    }

    function getConnection(){
        global $connection;
        var_dump($connection); // I get -> object(DatabaseManipulation)#1 (1) { ["connection":"DatabaseManipulation":private]=> NULL } NULL 
    }

}

Same thing happens when I instantiate an object $connection = new DatabaseManipulation(); . Am I doing something wrong? I want this to be done in OO way. Can anyone help me ?

Answer 1

You are using OO PHP not procedural. So change it to:

$this->connection = new mysqli("localhost", "root", "", "db");;

Answer 2

I want this to be done in OO way.

Then stop using global . You don't want to refer to a global state when you're in a class. A quick look to The Basics and you will find that a pseudo-variable $this is available inside your object.

class DatabaseManipulation
{
    private $connection;

    function __construct()
    {
        $this->connection = new mysqli("localhost", "root", "", "db");
    }

    function getConnection(){
        var_dump($this->connection);
    }
}

More about it:

1. Why is Global State so Evil?
2. What does the variable $this mean in PHP?