file_get_contents()函数为空

if(isset($_REQUEST['act'])){
    $act = $_REQUEST['act'];
}
if(isset($_REQUEST['filename'])){
    $filename = $_REQUEST['filename'];
}
$redirect = "index.php?path={$path}";
if(isset($act)){
    if($act == "createFile"){
        //创建文件
        $mes = createFile($path."/".$filename);
        alertMes($mes,$redirect);
    }elseif($act == "showContent"){
        //查看文件内容
        $content = file_get_contents($filename);
        highlight_string($content);
    }
}
————————————————————
<td>
<a href="index.php?act=showContent&path=<?php echo $path;?>&fileName=<?php echo $path."/".$val;?>"><img src="images/show.png" alt="" title="查看"/></a>
<a href="index.php?act=editContent&path=<?php echo $path;?>&fileName=<?php echo $path."/".$val;?>"><img src="images/edit.png" alt="" title="修改"/></a>
<a href="index.php?act=renameFile&path=<?php echo $path;?>&fileName=<?php echo $path."/".$val;?>"><img src="images/rename.png" alt="" title="重命名"/></a>
<a href="index.php?act=copyFile&path=<?php echo $path;?>&fileName=<?php echo $path."/".$val;?>"><img src="images/copy.png" alt="" title="复制"/></a>
<a href="index.php?act=moveFile&path=<?php echo $path;?>&fileName=<?php echo $path."/".$val;?>"><img src="images/cut.png" alt="" title="剪切"/></a>
<a href="#" onclick="delFile('<?php echo $path;?>','<?php echo $path."/".$val;?>')"><img src="images/delete.png" alt="" title="删除"/></a>
<a href="index.php?act=downFile&path=<?php echo $path;?>&fileName=<?php echo $path."/".$val;?>"><img src="images/download.png" alt="" title="下载"/></a>
</td>

php.ini配置：allow_url_fopen = On ，并且没有禁用此函数。

但是就是无法使用。提示：

Notice: Undefined variable: filename in

Warning: file_get_contents(): Filename cannot be empty in