The first problem is sending mail from PHP from a smpt server that requires authentication. The standard php.ini file does not allow that. Fortunately, there is fake sendmail for windows which is included in the xampp installation. This requires modification of both the php.ini [mail function] section as well as the sendmail.ini file where the server authentication information is stored. With these modifications, the php mail function will work!
The primary problem is how to configure an e-mail attachment with a proper calendar invitation. This was kindly explained at Exchange Core. This seems to work not only with messages sent to an Exchange server, but also when sent to standard imap servers when viewed with a client that understands what to do with .ics attachments, such as the iPad. Awesome!