Actually PHP dosent offer any viewstate mechanism, as far as i know , So what you can do is store this in some hidden field.The best way and my personal recommendation is to use a session variable for this purpose
http://www.w3schools.com/Php/php_sessions.asp
And if you need to solve this issue using javascript, You can use Cookies too
http://www.w3schools.com/js/js_cookies.asp
I have done this using jquery and javascript by setting a cookie, hence i dont want you to get confused with jquery plugin for cookie. You can do this in a much more simpler way using jquery plugin for cookie. Here's the code
HTML
<select id="selectCurrency">
<option value="1">US</option>
<option value="98">RS</option>
<option value="61">Ind</option>
</select>
jquery/javascript
$(document).ready(function(e){
var name = "Currency=";
var ca = document.cookie.split(';');
for(var i=0; i<ca.length; i++){
var c = ca[i].trim();
if (c.indexOf(name)==0) $('#selectCurrency').val(c.substring(name.length,c.length));
}
});
$('#selectCurrency').change(function(e){
var cookieVal = "Currency="+$(this).val();
document.cookie = cookieVal ;
});
Fiddle