Using wordpress to interact with a webapp that has a RESTful API

hi there, i'm not sure if i'm allowed to ask this question but i'll try anyway.
I'm using formidablepro as my webform builder and i've got a pretty good handle of it including custom coding and creating add-ons to it.
I want to experiment something more advanced. The ultimate objective is to get user to submit a form entry and then use that entry to create another entry in a webapp called workflowmax (which has a RESTful api http://www.workflowmax.com/api/overview?qsu=1).

I was wondering if you could point me to any resource that might help me get started on the part where i can use the $_POST data to interact with the API. If you know of an open PHP library that can handle the basic of this (including authentication, creating PUT/POST/GET requests, and processing XML which is the output from the API), please let me know.