php-PHP只能下载英文命名的图片，不能下载中文命名的图片怎么办？

<?php
    function down_file($file_name,$file_sub_dir){
        $file_name=iconv("utf-8","gb2312",$file_name);

        $file_path=$_SERVER['DOCUMENT_ROOT'].$file_sub_dir.$file_name;
        if(!file_exists($file_path)){
            echo "文件不存在";
            return;
        }
        $fp=fopen($file_path,"r");
        $file_size=filesize($file_path);
        /*if($file_size>30){
            echo "<script language='javascript'>window.alert('文件过大');</script>";
            return;
        }*/

        header("Content-type: application/octet-stream");
        header("Accept-Ranges: bytes");
        header("Accept-Length: $file_size");
        header("Content-Disposition: attachment; filename=".$file_name);

        $buffer=1024;
        $file_count=0;
        while(!feof($fp) && $file_size-$file_count>0){
            $file_data=fread($fp,$buffer);
            $file_count+=$buffer;
            echo $file_data;
        }

        fclose($fp);
    }

    down_file("answer.jpg","/http/");

?>

 down_file("answer.jpg","/http/");

down_file("答案.jpg","/http/");