PHPでファイル拡張子を取得する3つの方法は何ですか

<?php
function getExt1($filename)
{
         $arr = explode(&#39;.&#39;,$filename);
         return array_pop($arr);
}
$str="dir/upload.image.jpg";
echo getExt1($str);
?>

<?php
function getExt4($filename)
{
         $arr = pathinfo($filename);
         $ext = $arr[&#39;extension&#39;];
         return $ext;
}
$str="dir/upload.image.jpg";
echo getExt4($str);
?>

<?php
function getExt5($filename)
{
         $str = strrev($filename);
         return strrev(strchr($str,&#39;.&#39;,true));
}
$str="dir/upload.image.jpg";
echo getExt5($str);
?>

strchr(string,search,before_search);