The super murder caused by the bom header in PHP - the static files in the head are generated into the body

<?php
if (isset($_GET[&#39;dir&#39;])) { //设置文件目录 
  $basedir = $_GET[&#39;dir&#39;];
} else {
  $basedir = &#39;.&#39;;
}
 
$auto = 1;
checkdir($basedir);
 
function checkdir($basedir)
{
  if ($dh = opendir($basedir)) {
    while (($file = readdir($dh)) !== false) {
      if ($file != &#39;.&#39; && $file != &#39;..&#39;) {
        if (!is_dir($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=&#39;red&#39;>BOM found, automatically removed.</font>");
    } else {
      return ("<font color=&#39;red&#39;>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);
}
?>