Do i HAVE to use a helper file?

[eluser]Michael Wales[/eluser]
You are in an object oriented development environment - you need to tell PHP what class to find that particular function in. In this case, $this works just fine.

Code:

$data['variables'] = array("username"   => $username,
                           "password"   => hash("sha256", $password),
                           "user_valid" => $this->validate_username($username));

On a side note, you probably want to make that function private by prepending an underscore (function _validate_username). Otherwise it is accessible via the URI.