Variables from other page

Hey,

I'm planning a app which link you can find on an other page. How I can send values to the app? In PHP I would do it with HTTP Get or PHP Session.

An example:

Pages: first.com (PHP Page), second.com (Ext JS App)

Now I want send variables/values (i.e. username) from page first.com to second.com. What is a good resolution?