WordPress, Gravity Forms and php

I am developing a site and I am using my very basic knowledge of php to try and conquer Mount Everest. I was never supposed to be the developer, only the designer, and I am now in the deep end. Any help on this will greatly be appreciated…

I am trying to dynamically populate a field using Gravity Forms, and the support there is GREAT, but takes time and I am running out of time, so I am trying anything at the moment.

The function to dynamically populate a field :

add_filter('gform_field_value_your_parameter', 'my_custom_population_function');
function my_custom_population_function($value){
    return 'boom!';
}

This works and I get boom! in my field… Now I am calling the function like this :

$unique_id = 'WP11003';
my_custom_population_function($unique_id);'

And here is the customised function :

add_filter(‘gform_field_value_your_parameter’, ‘my_custom_population_function’);
function my_custom_population_function($value){
echo $value;
return $value;
}`

Now what happens is that I get the correct value echoed, but nothing is returned to the form. If I put :

$$test = 'WP11013';
return $test;

Then it works, but as soon as I try to return the value that was passed to this function it doesn’t work at all.

I know this is very hard based on the snippet of code I gave, but any help will be really appreciated.

Thanks in advance!