无法使用Mysqli选择数据库

When I use mysql all works well but since I changed to mysqli it's not working at all. I connect I don't get the database I tried all day.

$u_name = $_SESSION['username'];

$conn=mysqli_connect($mysql_server, $mysql_username, $mysql_password,  
    $mysql_table) or die("<h2>Connection Failed</h2>");
mysqli_select_db($conn, $mysql_table);

$query="SELECT * FROM users WHERE username='{$_SESSION['username']}' 
    limit 1";

$result = mysqli_query($conn, "SELECT * FROM users WHERE   
    username='{$_SESSION['username']}' limit 1") or 
    die(mysqli_error($conn)); 

if($result->connect_errno > 0){
    die('error [' . $db->connect_error . ']');
}

$data = mysqli_fetch_array( $result );
$id=$data['id'];