Outputting a PHP echo in a hidden field

Is there an easy way to get this PHP echo to output in a hidden field?

<?php echo $visited_pages_string  ?>

The tooltip next to the "Allow fields to be populated dynamically" seems to indicate I can insert a shortcode there; but I tried tried putting it in a hidden field's default value as both PHP code and a shortcode, but that didn't work.

@David Thanks!

Forgive me, but I'm not a PHP programmer. I believe I understand that I need replace "date" in the example code on that page so it looks like this:

<?php
add_filter("gform_field_value_url", "populate_url");
function populate_url($value){
   return "10/10/2010";
}
?>

But I'm not sure what I would put in

$value

or

return "10/10/2010";

I added this code to functions.php

add_filter("gform_field_value_url", "populate_url");
function populate_url($value){
   return $visited_pages_string;
}

and created a hidden field called "url" a chcked "Allow field to be populated dynamically" and entered the parameter name "url" and it's just outputting an empty value in the <input> tag.

That didn't work either. :(