Help with PHP / MD5 hash

I need some help with some custom code... Now I'm not a coder, but I dabble...

What we are trying to accomplish is as follows..

Clients lands on page... Formidable Pro form appears with the fields for Amount, and invoice number... (Hidden fields for other data required like date/time etc)...

There is one field which concatenates a bunch of other fields and submits the data as an md5 Hash...

The support folks @ formidable have given me some pointers as far as where to look, but I'm not quite sure if I understand where to go next...

The only thing I need help with is to "MD5" a hidden field... I'm told that can be done by PHP... But I don't know enough to do that... Would it be a snippet of code embedded within the page ? The support team's direction seems to indicate it would be a fairly simple php snippet to convert the field to md5... but from what I can tell, running php from within the page is not so straightforward ? Do we create the smippet of code which uses the data from one field, md5's it, and then passed it onto another? Not sure how this would play out...

I'd also consider outsourcing this to someone... If someone would like to give me a quote, that would be cool too... But ideally I would like to learn how to do this for my own educational purposes.