Like the string.* filters, the convert.* filters perform actions similar to their names. The convert filters were added with PHP 5.0.0. For more information on a given filter, refer to the manual page for the corresponding function.
   convert.base64-encode and
   convert.base64-decode
   Use of these filters are equivalent to processing all stream data through
   the base64_encode() and base64_decode()
   functions respectively.
   convert.base64-encode supports parameters given as
   an associative array.  If line-length is given, the
   base64 output will be split into chunks of line-length
   characters each.  If line-break-chars is given, each
   chunk will be delimited by the characters given.  These parameters give the
   same effect as using base64_encode() with
   chunk_split().
  
Example #1 convert.base64-encode & convert.base64-decode
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.base64-encode');
fwrite($fp, "This is a test.\n");
fclose($fp);
/* Outputs:  VGhpcyBpcyBhIHRlc3QuCg==  */
$param = array('line-length' => 8, 'line-break-chars' => "\r\n");
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.base64-encode', STREAM_FILTER_WRITE, $param);
fwrite($fp, "This is a test.\n");
fclose($fp);
/* Outputs:  VGhpcyBp
          :  cyBhIHRl
          :  c3QuCg==  */
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.base64-decode');
fwrite($fp, "VGhpcyBpcyBhIHRlc3QuCg==");
fclose($fp);
/* Outputs:  This is a test.  */
?>
   convert.quoted-printable-encode and
   convert.quoted-printable-decode
   Use of the decode version of this filter is equivalent to processing all stream 
   data through the quoted_printable_decode() functions.
   There is no function equivalent to convert.quoted-printable-encode.
   convert.quoted-printable-encode supports parameters given as
   an associative array.  In addition to the parameters supported by 
   convert.base64-encode, convert.quoted-printable-encode
   also supports boolean arguments binary and
   force-encode-first.
   convert.base64-decode only supports the
   line-break-chars parameter as a type-hint
   for stripping from the encoded payload.
  
Example #2 convert.quoted-printable-encode & convert.quoted-printable-decode
<?php
$fp = fopen('php://output', 'w');
stream_filter_append($fp, 'convert.quoted-printable-encode');
fwrite($fp, "This is a test.\n");
/* Outputs:  =This is a test.=0A  */
?>