PHP如何去除BOM

/* 
 +------------------------------------------------------------------------------------------- 
 + Title : 去掉BOM头方法 
 + Author : hello_sgw 
 + Version : V1.0.0.1 
 + Initial-Time : 2017-08-12 15:18
 + Last-time : 2017-08-12 16:01
 + Desc : 
 +------------------------------------------------------------------------------------------- 
*/

BOM --Byte Order Mark，中文名译作“字节顺序标记”，在utf-8编码文件中BOM在文件头部，占用三个字节，用来标示该文件属于utf-8编码，
现在已经有很多软件识别bom头，但是还有些不能识别bom头，比如PHP就不能识别bom头，这也是用记事本编辑utf-8编码后执行就会出错的原因了。

<?php
if (isset($_GET['dir'])) { //设置文件目录 
 $basedir = $_GET['dir'];
} else {
 $basedir = '.';
}
 
$auto = 1;
checkdir($basedir);
 
function checkdir($basedir)
{
 if ($dh = opendir($basedir)) {
 while (($file = readdir($dh)) !== false) {
 if ($file != '.' && $file != '..') {
 if (!is_dir($basedir . "/" . $file)) {
 echo "filename: $basedir/$file " . checkBOM("$basedir/$file") . " <br>";
 } else {
 $dirname = $basedir . "/" . $file;
 checkdir($dirname);
 }
 }
 }
 closedir($dh);
 }
}
function checkBOM($filename)
{
 global $auto;
 $contents = file_get_contents($filename);
 $charset[1] = substr($contents, 0, 1);
 $charset[2] = substr($contents, 1, 1);
 $charset[3] = substr($contents, 2, 1);
 if (ord($charset[1]) == 239 && ord($charset[2]) == 187 && ord($charset[3]) == 191) {
 if ($auto == 1) {
 $rest = substr($contents, 3);
 rewrite($filename, $rest);
 return ("<font color='red'>BOM found, automatically removed.</font>");
 } else {
 return ("<font color='red'>BOM found.</font>");
 }
 } else
 return ("BOM Not Found.");
}
 
function rewrite($filename, $data)
{
 $filenum = fopen($filename, "w");
 flock($filenum, LOCK_EX);
 fwrite($filenum, $data);
 fclose($filenum);
}