Powerful file type detection function php code

/**
 * @access public
 * @param string filename file name
 * @param string limit_ext_types allowed file types, types surrounded by | such as: |gif|txt|
 * @return string
 * by http://bbs. it-home.org
*/
function check_file_type($filename, $limit_ext_types = ''){
 $extname = strtolower(substr($filename, strrpos($filename, '.') + 1));
 if ($limit_ext_types &&
 stristr($limit_ext_types, '|' . $extname . '|') === false){
  return '';
 }

 $str = $format = '';

 $file = @fopen($filename, 'rb');
 if ($file){
  $str = @fread($file, 0x400); // 读取前 1024 个字节
  @fclose($file);
 }
 else{
  $format=$extname;
 }

 if ($format == '' && strlen($str) >= 2 ){
  if (substr($str, 0, 4) == 'MThd' && $extname != 'txt'){
   $format = 'mid';
  }
  elseif (substr($str, 0, 4) == 'RIFF' && $extname == 'wav'){
   $format = 'wav';
  }
  elseif (substr($str ,0, 3) == "xFFxD8xFF"){
   $format = 'jpg';
  }
  elseif (substr($str ,0, 4) == 'GIF8' && $extname != 'txt'){
   $format = 'gif';
  }
  elseif (substr($str ,0, 8) == "x89x50x4Ex47x0Dx0Ax1Ax0A"){
   $format = 'png';
  }
  elseif (substr($str ,0, 2) == 'BM' && $extname != 'txt'){
   $format = 'bmp';
  }
  elseif ((substr($str ,0, 3) == 'CWS' || substr($str ,0, 3) == 'FWS')
  && $extname != 'txt'){
   $format = 'swf';
  }
  elseif (substr($str ,0, 4) == "xD0xCFx11xE0"){   // D0CF11E == DOCFILE == Microsoft Office Document
   if (substr($str,0x200,4) == "xECxA5xC1x00"
   || $extname == 'doc'){
    $format = 'doc';
   }
   elseif (substr($str,0x200,2) == "x09x08" || $extname == 'xls'){
    $format = 'xls';
   }
   elseif (substr($str,0x200,4) == "xFDxFFxFFxFF"
   || $extname == 'ppt'){
    $format = 'ppt';
   }
  }
  elseif (substr($str ,0, 4) == "PKx03x04"){
   $format = 'zip';
  }
  elseif (substr($str ,0, 4) == 'Rar!' && $extname != 'txt'){
   $format = 'rar';
  }
  elseif (substr($str ,0, 4) == "x25PDF"){
   $format = 'pdf';
  }
  elseif (substr($str ,0, 3) == "x30x82x0A"){
   $format = 'cert';
  }
  elseif (substr($str ,0, 4) == 'ITSF' && $extname != 'txt'){
   $format = 'chm';
  }
  elseif (substr($str ,0, 4) == "x2ERMF"){
   $format = 'rm';
  }
  elseif ($extname == 'sql'){
   $format = 'sql';
  }
  elseif ($extname == 'txt'){
   $format = 'txt';
  }
 }

 if ($limit_ext_types &&
 stristr($limit_ext_types, '|' . $format . '|') === false){
  $format = '';
 }

 return $format;
}