CIiBug? Url Encode /Url Decode problem in url Query Strings

[eluser]mindprojects[/eluser]
A better explanation:

1) i have an input text into a form(name='q')
2) i submit the form after typing 'a + b', to the same controller
3) if the validation run === TRUE then i build my new url and redirect to this url.
To send the string i need to urlencode/rawurldecode,so that i do:

urlencode(a + b)

Then i redirect to :
controller/function/q/a+%2_B+b (to avoid the refresh multiple post problem)

4)Into the destination controller to get the parameter i do
urldecode($this->uri->segment(4),but i can get only 'a b'(not plus at all)

I found a solution here using GET!
http://ellislab.com/forums/viewthread/56389/#277621,but i can't understand why it's not working by the uri segment way.