Code to Populate a Hidden Field

I am trying to populate a hidden field using the gform_pre_submission hook. I am placing this code in custom_functions.php in my Thesis site. Here is URL to form:
http://www.halloweenfun.net/submit-site-review/

Here is the code, but it is invalid, because it is "crashing" my custom_functions.php.
Any help?

add_action("gform_pre_submission", "create_attribution");

function create_attribution($form){
if($form["id"] == '1'){
$hidden-byline = "Submitted by " . $_POST["input_4"] . ", from ". $_POST["input_5"] . ", who " . $_POST["input_6"] . " affiliated with the site.";
$_POST["input_8"] = $hidden-byline;
}
return $form;
}

Thank you David!

That's what I get for trying to "impersonate a PHP programmer." But the code itself (after fixing the variable name) ... actually worked! Hooray!!!!