Taking a bunch of the earlier comments together, you can synthesize a nice short and reasonably efficient quoted_printable_encode function like this:

Note that I put this in my standard library file, so I wrap it in a !function_exists in order that if there is a pre-existing PHP one it will just work and this will evaluate to a noop.

<?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 aren't encoded as =20 though.
  */
  function quoted_printable_encode($string) {
    return preg_replace('/[^\r\n]{73}[^=\r\n]{2}/', "$0=\r\n", str_replace("%","=",str_replace("%20"," ",rawurlencode($string))));
  }
}
?>

Regards,
Andrew McMillan.