php从给定url获取文件扩展名的方法

<?php
/**
 * 给定url，获取文件后缀
 * @param string $url
 * @return string
 */
function getUrlPostfix ($url)
{
  $url_arr = explode(‘.‘, $url);
  $postfix = $url_arr[count($url_arr) - 1];
  $substr = substr($postfix, 0, 3);
  return $substr;
}
$url = "http://www.jb51.net/index.html?id=1";
$str = getUrlPostfix($url);
echo $str . "\n";