passing data from custom web site to Psytoolkit survey

Hi there,
I wonder if someone would be good enough to help me with the following problem.

I need to redirect a subject from Psytoolkit to my own custom program (mix of html, php & Javascript), then when the subject has carried out the customised task, pass them back to the survey for the last few questions.

The only catch is that I need to pass an item of data, namely the subject identifier from Psytoolkit, to my own program (which I have succeeded in doing), then pass the subject identifier back to Psytoolkit by inserting the variable name into the 'In:' textbox.

How would I go about passing data back to Psytoolkit? I would be using HTML or PHP to do this, do I use POST, GET or some other method?

I need to be able to pass on the subject identifier so that I can tie in all the data together for each subject.

Thanks & regards,
Shaun