警告：mysqli :: query（）：无法获取mysqli

Question

我不明白错误：

  

＆＃34;警告：mysqli :: query（）：无法获取mysqli   第15行的var / www / html /.../addUser.php错误：未添加用户＆＃34;

有人有解决方案的想法吗？

addUser.php

<?php

    ini_set('display_errors', 1);
    ini_set('display_startup_errors', 1);
    error_reporting(E_ALL);

    include_once('config.php');
    $name = $_POST['name'];
    $firstname= $_POST['firstname'];
    $email = $_POST['email'];


    $sql = "INSERT INTO user(name, firstname, email) VALUES('$name', '$firstname', '$email')";

    if($connection->query($sql) === TRUE){
        echo 'User has been added';
    }
    else
    {
        echo 'error : User not added';
    }
?>

的config.php

?php

    define('DB_HOST', 'localhost');
    define('DB_NAME', 'db');
    define('DB_USERNAME', 'user');
    define('DB_PASSWORD', '');

    $connection = new mysqli(DB_HOST, DB_USERNAME, DB_PASSWORD, DB_NAME) or die("Could not connect to database");


    if(mysqli_connect_errno()){
        echo('Connection fail'.mysqli_connect_error());
        exit();
    }


    $connection->close();

?>