In addition to the previous comment, php5-idn has merged into the php5-intl package. Use apt-get install php5-intl on ubuntu to use these idn features.
In addition to the previous comment, php5-idn has merged into the php5-intl package. Use apt-get install php5-intl on ubuntu to use these idn features.Keep in mind that a php module is required to use these functions (which may not be present on your shared hosting).
php5-idn - PHP API for the IDNA library (e.g. in Debian/Ubuntu)