Library for International Domain Names (IDNA) 2008


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International Domain Names PHP library

Introduction & terminology

Historically, we've been able to use domain names composed only by ASCII characters (for instance:

A new technique, called Internationalized Domain Names (IDN for short), allows you to use most of the Unicode characters, so that you can have for instance www.例.中国.

To grant compatibility with all the existing software that makes internet work, domain names containing non-ASCII characters are represented in Punycode, a special format that uses ASCII-only characters.


The generation of Punycode starting from an IDN should be case insensitive: browsing to should be the same as browsing to www.Example.COM.

In PHP, converting strings to lower case is as easy as calling strtolower, but this function does not work with characters outside the ASCII characters (in fact, it may mess up the IDN names). If you have the mbstring PHP extension, you may think to use the mb_strtolower PHP function it offers.

By the way, even mb_strtolower isn't a good choice, for these reasons:

  1. the mbstring PHP extension may not be available
  2. the mb_strtolower behaviour changes across different PHP versions (for instance, Ԩ is correctly converted to ԩ for PHP 7.0, but prior versions kept Ԩ)
  3. mb_strtolower does not translate a lot of Unicode characters that are suggested by the standards

Unicode offers a mapping table with the recommended mapping (for instance, case normalization like A to a, but also to .).


There are two standards that define the mapping that should be applied to IDN, IDNA2003 and IDNA2008. IDNA2008 is backward compatible with IDNA2003, but there are some incompatible differences.

For instance, IDNA2003 required that ß mapped to ss, whereas IDNA2008 allows the usage of ß. So, older browsers and client softwares resolved www.schloß.com to the Punycode corresponding to, whereas newer browsers resolve it to the Punycode of www.schloß.com.

Since the resulting Punycode is different (it's called deviation), this lead to big security issues, and you need to know that a domain name is deviated.

Advantages of this library

  • no dependencies from any PHP extension
  • not dependent from any other PHP library
  • consistency across different PHP versions
  • results are granted to follow the standards (it's not just a bare multibyte to punycode conversion library)
  • designed with speed in mind
  • compatible with PHP 5.3

Sample usage

use MLocati\IDNA\DomainName;

require_once 'autoload.php'; // Not required if you use composer

$domain = \MLocati\IDNA\DomainName::fromName('www。schloß.COM');

echo "Name: ", $domain->getName(), "\n";
echo "Punycode: ", $domain->getPunycode(), "\n";
echo "Deviated: ", $domain->isDeviated() ? 'yes' : 'no', "\n";
echo "Deviated Name: ", $domain->getDeviatedName(), "\n";
echo "Deviated Punycode: ", $domain->getDeviatedPunycode(), "\n";


Name: www.schloß.com
Deviated: yes
Deviated Name:
Deviated Punycode: