echo $entry causes Warning:Cannot modify header information

I just bought this plugin and I can't get it working the way I want yet.

Here's what I want to do:
1 - Create a from in the sidebar and redirect it's results to Search.php when hitting the Send button
2 - In Search.php use some data from the form to create a Buy button which will open Confirm.php
3 - In Confirm.php use data from the form and Search.php to create a new form.

Unfortunately, I can't pass step 1 because when I click the send button I have the following error

2012-07-03
Warning: Cannot modify header information - headers already sent by (output started at /Applications/MAMP/htdocs/NihadCar/wp-content/themes/CaRental/functions.php:42) in /Applications/MAMP/htdocs/NihadCar/wp-content/plugins/gravityforms/form_display.php on line 83

2012-07-03 is the value I'm looking for

I simply added this to the functions file to start testing:

function set_post_content($entry, $form){
	echo $entry["1"];
}
add_action("gform_after_submission_3", "set_post_content", 10, 2);

If I replace echo with return no errors but no output either!

I'm testing locally so I can't give any DEMO
I'm using Mamp Pro with Lion

Maybe I'm wrong, but i thought I could use gform_after_submission to display data from the form !

In the form setting I used the redirection option to call Search.php

In search.php I have a variable and I want it to have the value of the field $entry[1]

I have managed to do the job by passing the values by Query String but I really don't wont the URL to get ugly, so If I could use something else it would be great.

Thanks for your answer

Yes query string works fine but I don't want all the data to appear in the URL adress.

Yes I want to get the data after the submission, I thought that I can create a global variable in the Filter Call and use that global variable elsewhere, I think I don't understand how to use filters.