根据HTML表单中选择的位置自动生成密码

Question

I am currently working on a website that is intended to offer certain services depending upon the location of the user. Now the problem is: How do I auto generate the Pin Code text field depending upon the location selected in the drop down list? My project is based on PHP

http://pastebin.com/5bcbDJ1u - here is the html form

http://pastebin.com/AjzD1GL2 - here is the PHP file used in the action tag of form