But I guess that you want to use it as a “HTML based web service”. PHP with curl should be fine, so you can fetch the registration page within your PHP script and evaluate it before you generate a new page for your user. The script could also receive the registration requests, validate them and call then the registration plugin. So this PHP script would work as a very intelligent proxy. And you don’'t need to open the Openfire admin console (port 909x) for everyone.
I try to avoid using PHP where this is possible. Not sure how you want to parse and validate the input if you don’'t write your own PHP script, the PHP online documentation about libcurl contains already some samples.