PHPresolver is a DNS resolver library written in pure PHP. It may help you code email address validity verification under the Win32/BeOS port of PHP because the current version has no support for DNS-related functions such as getmxrr() or checkdnsrr(). In addition, the library partially supports IPv6 address resolution.
Whoisdomaincheckup is a Web-based (PHP) application that can check the registration status of a domain name. In particular, it can check whether a domain name is currently free for registration. It doesn't use any exec or system commands, but instead uses the fopen command to access the Whois servers.
PHP Net_IDNA is a class to convert between the Punycode and Unicode formats. Punycode is a standard described in RFC 3492 and part of IDNA (Internationalizing Domain Names in Applications [RFC3490]) . This class allows PHP scripts to convert these domain names without having one of the PHP extensions installed. It supports both IDNA 2003 and IDNA 2008.