php怎么写倒序输出的方法

$str = "abcdef";
for (
$i=1;
$i<=strlen($str);
$i++)
{
echo substr($str , -$i , 1)
}

// 正序
foreach($files as $file_num => $file) {
if(is_file($directory.$file)){
//$file = iconv("gb2312","UTF-8",$file); //或者 iconv("gb2312","UTF-8",$value);
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$date = substr($file,0,9);
echo '<li class="world-cup-".$date.'">';
echo '<a href="'.$directory.$file.'" rel="worldcup" title="卡塔尔世界杯赛事".$date.'">';
echo '<img src="'.$directory.$file.'" alt="">";
echo '</a>';
echo '</li>';
}
}