renameFile($oldname,$newname)中的$oldname值为空 传递不上

function renameFile($oldname,$newname){
    echo $oldname,$newname;
}

elseif ($act=="renameFile"){
            //完成重命名
            $str=<<<EOF
                <form action="index.php?act=doRename" method="post" >
            请填写新文件名：<input type="text" name="newname" placeholder="重命名" /><br/>
            <input type='hidden' name='filename' value='{$filename}' />
            <input type="submit" value="重命名">
            </form>
EOF;
            echo $str;
        }elseif ($act=="doRename"){

            //实现重命名的操作
            $newname=$_REQUEST['newname'];
            renameFile($filename, $newname);
        
        }