HTML Form to Create New Client

[MarshallCCA]

Ok, so I am making a web app that contains an HTML form which gathers
data and sends it to our cloud systems (Solve360 CRM, FreshBooks and
Google Docs)...I've got the CRM portion working by sending the data
from the HTML form to a php file that handles that data.

My question now is: How do I take that same data from the table and
get it to translate into XML that the FreshBooks API understands?

I have found the code for client.create but how do I populate the
fields with my form data? ie: <first_name>name from form goes here</
first_name> and so on?

and where do I put the cURL request? do I need to make a whole new XML
file that specifically handles all of the FB data and call that from
my form or what?

Sorry if I'm not making much sense, this is my first time working with
both PHP and XML!