我的查询说，即使已经在php中选择了我的数据库，也没有选择数据库

Question

我收到的错误是这样的：

SELECT * FROM tenants WHERE ic='123456789'

未选择数据库。

如果我已经选择了我的数据库，为什么它没有选择数据库。 这是我的PHP代码：

require "../config/database.php";

$error_text = "Oops! This is a registered identification number! Please check in the tenants list.";

// checking ic already exists
$sql="SELECT * FROM tenants WHERE ic='$ic' ";

$result=mysqli_query($conn,$sql);

if($error_mysql){
        echo $error_mysql;
    }

return ($result) ? $error_text : "valid";`

这是我的database.php代码：

/* Define your data to access MySQL database */
    define("REG_USER", "root");                     // your username
    define("REG_SERVER", "localhost");                      // your host
    define("REG_PASSWORD", "");                 // your password
    define("REG_DATABASE", "rsystem");                  // your database

/* Variables */
    $error_exists = false;
    $error_mysql = "";

    /* Connection to DB */
    /* Constants, that defined in action.php, are used */
    $conn = mysqli_connect(REG_SERVER, REG_USER, REG_PASSWORD, REG_DATABASE);
    if (mysqli_connect_error()) {
        $error_mysql = ("Error connecting to database (" . mysqli_connect_errno() . ") ". mysqli_connect_error());

    }
    mysqli_set_charset($conn, 'utf8');

感谢。