Chome Extension: Send request to background page via a javascript on server
===

I am developing a chrome extension which injects an iframe in a particular webpage as soon as it loads. The iframe loads a form(form.php) hosted on localhost. All the information entered on the form is processed by 'script.js' on the localhost. 
The Php File has:

    <script src="script.js" type="text/javascript"></script>
There is a submit button on the php file. When it is clicked I want to pass a message and some data to the extension's background page like:

    chrome.extension.sendRequest({'name':'email', value:email});

but i get an error saying:

    Uncaught TypeError: Cannot call method 'sendRequest' of undefined 

What I want is, once the submit button is clicked, all the data that is processed is sent to the chrome extension background page.

Please suggest me any method that can help me do that?