HTTP错误：HTTP标头后没有数据

Question

您好我正在尝试查询数据库并将值返回给客户端。所以这是我的代码有两个功能。该速率应该在db中得到速率。

    public function connect($description){

    //creating connection
    $con = new mysqli('localhost','root','','ci_vetro');
    //mysql_connect('localhost','root','') or die("Connection Error: ".mysql_error());
    //mysql_select_db('ci_vetro');

    //check connection
    if($con->connect_error):
        die('Connection Error: '. $con->connect_error);
    endif;

    $sql= "SELECT * from. exchange_rates where description=.'$description'.";
    $result= $con->query($sql);

    if($result->num_rows > 0):
        while($row = $result->fetch_assoc()) :
           $rate=$row["rate"]. "<br>";
        endwhile;
    else:
        echo "0";
    endif;

    $con->close();
    return $rate;

}
    public function getCurrency($currency) {

        $rate=connect($currency);

    switch ($currency){
        case 'USD':
            return int($rate);
        break;
        default:
          return 0;
        break;
    }
}