How do I securely submit "Username" and "Password" from an applet to a server side php3 program?

One way to do this operation is that to make an applet to serverside program calls the program from applet using the Https protocol, for example:
URL servlet = new URL("https://yourservername/yourprogramname?param=value&......");
AppletContext ac = getAppletContext();
ac.showDocument(servlet);