protected function _standardize($string)
{
// transform CR or LF to CRLF
$string = preg_replace('~=0D(?!=0A)|(?<!=0D)=0A~', '=0D=0A', $string);
// transform =0D=0A to CRLF
$string = str_replace(array("\t=0D=0A", ' =0D=0A', '=0D=0A'), array("=09\r\n", "=20\r\n", "\r\n"), $string);
switch ($end = ord(substr($string, -1))) {
case 0x9:
$string = substr_replace($string, '=09', -1);
break;
case 0x20:
$string = substr_replace($string, '=20', -1);
break;
}
return $string;
}