A function that QP-encodes an input string (written for PHP < 5.3) and
wordwraps it at the same time, in order to avoid classification according to the MIME QP LONG LINE rule of SpamAssassin.  Thanks for Matt Jeffers to point out errors in the below quoted_printable script!

<?php
function quoted_printable_encode($input, $line_max = 75) {
   $hex = array('0','1','2','3','4','5','6','7',
                          '8','9','A','B','C','D','E','F');
   $lines = preg_split("/(?:\r\n|\r|\n)/", $input);
   $linebreak = "=0D=0A=\r\n";
   /* the linebreak also counts as characters in the mime_qp_long_line
    * rule of spam-assassin */
   $line_max = $line_max - strlen($linebreak);
   $escape = "=";
   $output = "";
   $cur_conv_line = "";
   $length = 0;
   $whitespace_pos = 0;
   $addtl_chars = 0;

   // iterate lines
   for ($j=0; $j<count($lines); $j++) {
     $line = $lines[$j];
     $linlen = strlen($line);

     // iterate chars
     for ($i = 0; $i < $linlen; $i++) {
       $c = substr($line, $i, 1);
       $dec = ord($c);

       $length++;

       if ($dec == 32) {
          // space occurring at end of line, need to encode
          if (($i == ($linlen - 1))) {
             $c = "=20";
             $length += 2;
          }

          $addtl_chars = 0;
          $whitespace_pos = $i;
       } elseif ( ($dec == 61) || ($dec < 32 ) || ($dec > 126) ) {
          $h2 = floor($dec/16); $h1 = floor($dec%16);
          $c = $escape . $hex["$h2"] . $hex["$h1"];
          $length += 2;
          $addtl_chars += 2;
       }

       // length for wordwrap exceeded, get a newline into the text
       if ($length >= $line_max) {
         $cur_conv_line .= $c;

         // read only up to the whitespace for the current line
         $whitesp_diff = $i - $whitespace_pos + $addtl_chars;

        /* the text after the whitespace will have to be read
         * again ( + any additional characters that came into
         * existence as a result of the encoding process after the whitespace)
         *
         * Also, do not start at 0, if there was *no* whitespace in
         * the whole line */
         if (($i + $addtl_chars) > $whitesp_diff) {
            $output .= substr($cur_conv_line, 0, (strlen($cur_conv_line) - 
                           $whitesp_diff)) . $linebreak;
            $i =  $i - $whitesp_diff + $addtl_chars;
          } else {
            $output .= $cur_conv_line . $linebreak;
          }

        $cur_conv_line = "";
        $length = 0;
        $whitespace_pos = 0;
      } else {
        // length for wordwrap not reached, continue reading
        $cur_conv_line .= $c;
      }
    } // end of for

    $length = 0;
    $whitespace_pos = 0;
    $output .= $cur_conv_line;
    $cur_conv_line = "";

    if ($j<=count($lines)-1) {
      $output .= $linebreak;
    }
  } // end for

  return trim($output);
} // end quoted_printable_encode
?>