致命错误:在布尔值上调用成员函数query()

Question

我正在写一个简单的数据库来练习,但我一直收到这个错误:

致命错误:在第43行的/Applications/XAMPP/xamppfiles/htdocs/index.php中调用boolean上的成员函数query()

查询行是第43,但我不明白为什么它是一个布尔值既不$conn或者$db_handle是false,这是为什么？

    <?php

    $server = "localhost";
    $database = "motocross_db";
    $username = "root";
    $password = "";

    $conn = new mysqli($server, $username, $password);
    $db_handle = $conn->select_db($database);

    if ($db_handle)
    {
        if ($_SERVER['REQUEST_METHOD'] == 'POST')
        {
            // upload
        }

        $enrollments_sql = "SELECT
                               users.first_name,
                               users.last_name,
                               registrations.attendance_date,
                               registrations.attendance_location,
                               bikes.bike_brand,
                               bikes.engine_size,
                               bikes.bike_type
                        FROM users
                        INNER JOIN registrations
                           ON users.user_id = registrations.user_id
                        INNER JOIN bikes
                           ON registrations.bike_id = bikes.bike_id";

        $enrollments_result = $db_handle->query($enrollments_sql);
    ?>

Answer 1

更改

$db_handle->query($enrollments_sql);

至

$conn->query($enrollments_sql);// use $conn here

因为mysqli_query()需要第一个参数作为数据库连接

阅读http://php.net/manual/en/mysqli.query.php