Return Script 2CO & Pass It Further.

I am a vendor with [login to view URL] (2CO). I need a return script from where we can receive the custom parameters from 2checkout and use it to make HTTP post to my sever. I don't want to directly pass the parameters to the server as some of the parameter format which are required by server are already shared by 2CO so hence i cant use those parameters since starting. When a custom parameter is received from 2Checkout, The parameter format has to be changed and then make a http post to the server. Example:- A parameter is received from the 2CO in x_username format. The value of this format has to be received and be posted to the server in x_login format. The value of x_login should be same as x_username. We will be receiving around 10-15 parameters from 2CO but only 5 of them has to be posted. Only one parameter format will be changed and in one case values will be changed while rest all format and values will be passed as it is. For security reasons, the script should receive parameters only when one of the parameter has a particular value. This is known as md5 in 2CO. Payment will be made instantly once tested successfully.