I also designed a "php session value setter" solution by myself (similar to Luke Dennis' solution. No big deal here), but after setting my session value, my needs were "jumping onto another .php file". Ok, I did it, inside my jquery code... But something didn't quite work...
My problem was kind of easy:
-After you "$.post" your values onto the small .php file, you should wait for some "success/failure" return value, and ONLY AFTER READING THIS SUCCESS VALUE, perform the jump. If you just immediately jump onto the next big .php file, your session value might have not become set onto the php sessions runtime engine, and will you probably read "empty" when doing $_SESSION["my_var"]; from the destination .php file.
In my case, to correct that situation, I changed my jQuery $.post code this way:
$.post('set_session_value.php', { key: 'keyname', value: 'myvalue'}, function(ret){
if(ret==0){
window.alert("success!");
location.replace("next_page.php");
}
else{
window.alert("error!");
}
});
Of course, your "set_session_value.php" file, should return 'echo "0"; ' or 'echo "1"; ' (or whatever success values you might need).
Greetings.