How to return the size of directory even if the directory is symlink with PHP?

getFolderSize.php

<?php
/**
* @author mocanuga  
* @desc Return the size of directory in bytes. Tested on CentOS 5.11 Final
* @return int
*/ 
function getFolderSize($folder = '') {
  if(empty($folder))
    return -1;
  $dir = escapeshellcmd($folder);
  if(!is_dir($dir))
    return -1;
  $cmd = is_link($dir) ? `du -sb $(readlink -f $dir)` : `du -sb $dir`;
  $res = explode("\t", $cmd);
  return (int)array_shift($res);
}