PHP获得文件的大小并转换格式

function tosize($bytes,$prec=2){
    $rank=0;
    $size=$bytes;
    $unit="B";
    while($size>1024){
        $size=$size/1024;
        $rank++;
    }
    $size=round($size,$prec);
    switch ($rank){
        case "1":
            $unit="KB";
            break;
        case "2":
            $unit="MB";
            break;
        case "3":
            $unit="GB";
            break;
        case "4":
            $unit="TB";
            break;
        default :

    }
    return $size." ".$unit;
 }