As this article says, there is no quoted_printable_encode function() in PHP: http://www.zend.com/manual/filters.convert.php
However there is a stream filter for quoted printable encoding. Here's an example function that produces output suitable for email and doesn't explicitly use external files (though it might do for strings over 2Mb due to the nature of the temp stream type):
<?php
function quoted_printable_encode($string) {
$fp = fopen('php://temp/', 'r+');
$params = array('line-length' => 70, 'line-break-chars' => "\r\n");
stream_filter_append($fp, 'convert.quoted-printable-encode', STREAM_FILTER_READ, $params);
fputs($fp, $string);
rewind($fp);
return stream_get_contents($fp);
}
echo quoted_printable_encode(str_repeat("hello there ", 50)." a=1\r\n")."\n";
?>
The filter needs to be restricted to STREAM_FILTER_READ because by default it will get filtered both going into and out of the stream, and will thus get encoded twice.
It should be much faster than using a PHP implementation of the same thing, though note that this will only work in PHP 5.1+.