I don't think you can do that exactly how you are suggesting.
If you want to save a variable in a file, you're going to have to open the file, and go line by line until you get to the line which has your variable. I don't think that's what you want to do.
You might want to do something more like this, storing the email address in a file specifically for that purpose:
$email = $_POST['email'];
$fh = fopen("includes/email.php","w");
Then you could include includes/email.php (instead of config.php) to get your email address.
Of course, you're going to want to make sure that no one else can submit the form.
I don't know how secure this is, so if anyone sees any issues with it, please point them out...