PHP - 在null上调用成员函数query() - 错误

Question

我在php中有以下代码用于连接到我的数据库:

<?php
class MY_SQL{
    private $username;
    private $password;
    private $conn;

    public function __construct($SERVERNAME){
        $this->username = "username";
        $this->password = "password";

        if($SERVERNAME == "data_"){
            $server = "Servername";
        }
        else {
            $server = $SERVERNAME;
        }

        // Create connection
        $conn = new mysqli($server, $this->username, $this->password);

        // Check connection
        if ($conn->connect_error) {
            die("Connection failed: " . $conn->connect_error);
        } 
    }

    public function SQLCommand($cmd) {
        if ( $this->conn->query($cmd) === TRUE ) {
            echo "New record created successfully";
        } else {
            echo "Error: " . $cmd . "<br>" . $conn->error;
        }
    }
}

$sql = "INSERT INTO _test(test1, test2) VALUES ('hello','hi');";

        $database = new MY_SQL("Servername");
        $database->SQLCommand($sql);
?>

我收到以下错误:

致命错误:在null上调用成员函数query()

出了什么问题？

Answer 1

$this->conn = $conn; in __construct()