Description:
------------
For some reason, PHP reports:

Call to undefined function: iconv()

from time to time. A little later, the function will work without a hitch, without any changes being made to the PHP configuration. 

Configure command is:

'./configure' '--with-kerberos' '--with-openssl' '--prefix=/usr/local' '--with-mysql=/usr' '--with-gd' '--with-imap' '--with-imap-ssl' '--with-xml' '--with-zlib' '--enable-bcmath' '--with-apxs=/usr/local/apache/bin/apxs' '--enable-track-vars' '--enable-versioning' '--with-ldap' '--with-mcrypt' '--with-mhash' '--with-pdflib=/usr' '--enable-shared-pdflib' '--with-swf' '--with-ttf' '--with-t1lib' '--enable-sysvsem' '--enable-sysvshm' '--enable-wddx' '--enable-memory-limit' '--enable-dbase' '--enable-ftp' '--with-curl'

Reproduce code:
---------------
Code which generates the error, but later works fine: 

function mime_decode($s) {
  $elements = imap_mime_header_decode($s);
  for($i = 0;$i < count($elements);$i++) {
    $charset = $elements[$i]->charset;
    $text =$elements[$i]->text;
    if(!strcasecmp($charset, "utf-8") ||
       !strcasecmp($charset, "utf-7"))
    {
      $text = iconv($charset, "iso-8859-1", $text);
    }
    $decoded = $decoded . $text;
  }
  return $decoded;
}

Note: The indata does not change either - same code with same data works fine from time to time - at other times not. Can't see any pattern to when it works and when it doesn't.

Expected result:
----------------
$text converted into iso-8859-1 format from UTF-8.

Actual result:
--------------
Call to undefined function: iconv()