if (!$connection) {
    // 连接数据库失败
    exit('<h1>连接数据库失败</h1>');
}

$query = mysqli_query($connection, 'select * from users;');

$query = mysqli_query($connection, 'delete from users where id = 4');

  $query = mysqli_query($connection, "insert into users values (null, '{$name}', '{$gender}', '{$birthday}', '{$avatar_target}')");

      $query = mysqli_query($connection, "update users set name = '{$user['name']}', gender = {$user['gender']}, birthday = '{$user['birthday']}', avatar = '{$user['avatar']}' where id = {$id}");

if (!$queryexit('<h1>查询失败</h1>');
}

while ($row = mysqli_fetch_assoc($query)) {
    var_dump($row);
}

mysqli_free_result($query);

mysqli_close($connection);