Another (improved) version of quoted_printable_encode(). Please note the order of the array elements in str_replace().
I've just rewritten the previous function for better readability.

<?php
if (!function_exists("quoted_printable_encode")) {
  /**
  * Process a string to fit the requirements of RFC2045 section 6.7. Note that
  * this works, but replaces more characters than the minimum set. For readability
  * the spaces and CRLF pairs aren't encoded though.
  */
  function quoted_printable_encode($string) {
        $string = str_replace(array('%20', '%0D%0A', '%'), array(' ', "\r\n", '='), rawurlencode($string));
        $string = preg_replace('/[^\r\n]{73}[^=\r\n]{2}/', "$0=\r\n", $string);

        return $string;
  }
}
?>