[#1] kloewer at gmail dot com [2011-10-17 15:08:03]

Use  iconv_mime_decode();

<?php
// This yields "Subject: Pr??fung Pr??fung"
echo iconv_mime_decode("Subject: =?UTF-8?B?UHLDvGZ1bmcgUHLDvGZ1bmc=?=",
                       0, "ISO-8859-1");
?>

http://php.net/manual/de/function.iconv-mime-decode.php

[#2] esvit at mail dot ru [2011-01-26 05:12:10]

That fixed the all caps issue:

<?php
function imap_utf8_fix($string) {
  return iconv_mime_decode($string,0,"UTF-8");
}
?>

[#3] number0 at tlen dot pl [2009-07-06 02:43:35]

string in uppercase error http://bugs.php.net/bug.php?id=44098
my function to decode string

<?php
function decodeMimeStr($string, $charset="UTF-8" )
{
      $newString = '';
      $elements=imap_mime_header_decode($string);
      for($i=0;$i<count($elements);$i++) 
      {
        if ($elements[$i]->charset == 'default')
          $elements[$i]->charset = 'iso-8859-1';
        $newString .= iconv($elements[$i]->charset, $charset, $elements[$i]->text);
      }
      return $newString;
}
?>

[#4] MetaYii [2009-06-22 13:24:50]

Be aware: imap_utf8 tends to return the string in uppercase.

[#5] Anonymous webmaster@luzon14DoTCom [2007-07-30 00:43:02]

You may also try this example:

$_subject = "=?iso-8859-1?b?THV6b26uMTQ=?= dot CoM";
echo utf8_decode(imap_utf8($_subject));

OUTPUT: Luzon?14 dot CoM

Regards,
Tuso/Nazareth

[#6] hans at lintoo dot dk [2005-09-19 04:42:29]

Also see: 
<?php
imap_mime_header_decode($sting);
?>
Don't try to decode subjects with this one... imap_mime_header_decode does a much better job at that.

[#7] marcelo at maccoy dot com dot br [2005-02-14 12:01:55]

# this script are correct with the right signs remove expression
# please, ignore the later.
function decode_utf8($str){
       # paterns
           $err="(=\?.{10,13}q\?_?|\?\=)";
           $pat = "/=([0-9A-F]{2})/"; 
           $cha="'.chr(hexdec(";
       # erase null signs in string
          $str=eregi_replace($err,"",$str);
       # to decode with eval and replace
          eval("\$str='".
                  preg_replace($pat,$cha."'$1')).'",$str)
                  ."';");
        # return
           return $str;
} 
#Note: you can to reduce this script in 3 lines.

[#8] aperez at informatica dot 24ruedas dot com [2004-06-10 12:05:01]

[#9] mogster_at_boomdesign.no [2002-10-31 03:53:13]

To get a complete decode to utf8 (which is the protocol with "national" characters), use these two:
$subject = "=?ISO-8859-1?Q?B=F8lla?=, med =?ISO-8859-1?Q?=F8l?= i baggen";
$subject = imap_utf8($subject);
$subject = utf8_decode($subject);
will return the subject in 8-bit ascii.
That is, both functions.

knutm :-)