class og $this preg_replace_callback()

Hvordan kan jeg bruge $this-> under preg_replace_callback()

"\$this->quoted_printable_encode_character"


<?php
  if(!class_exists("emaillib")) {
    class emaillib {

      function quoted_printable_encode_character($strMatches) {
        $strCharacter = $strMatches[0];
        return sprintf("=%02x", ord($strCharacter));
      }

      function quoted_printable_encode($strHtml) {
        $strHtml = preg_replace_callback("/[^\x21-\x3C\x3E-\x7E\x09\x20]/",
        "\$this->quoted_printable_encode_character", $strHtml);
        return preg_replace("/(.{73}[^=]{0,3})/", "$1"."=\r\n", $strHtml);
      }

    }
  }
?>