php利用for语句和substr()函数怎么实现字符串的反转

<?php
header('content-type:text/html;charset=utf-8');   
$str="123456789";
echo "字符串反转前：<br>";
var_dump($str);

$result = '';
$len = strlen($str);//获取字符串长度
for ($i = $len-1; $i>=0; $i--){
   $result.= substr($str,$i,1);
}
echo "字符串反转后：<br>";
var_dump($result);
?>

<?php
header('content-type:text/html;charset=utf-8');   
$str="A123456789B";
echo "字符串反转前：<br>";
var_dump($str);

$result = strrev($str);
echo "字符串反转后：<br>";
var_dump($result);
?>