1. why is used defined and not isset() for variables? (empty is better choise in fact)
   if(!defined($SIGNUP_BEGIN))

2. at the end of template is correct template array used:

$SIGNUP_TEMPLATE['start'] = $SIGNUP_BEGIN;
$SIGNUP_TEMPLATE['end'] = $SIGNUP_END;
$SIGNUP_TEMPLATE['body'] = $SIGNUP_BODY;
$SIGNUP_TEMPLATE['extended-user-fields'] = $SIGNUP_EXTENDED_USER_FIELDS;
$SIGNUP_TEMPLATE['coppa']= '';

but inside signup.php are variables used:
$text = $tp->parseTemplate($SIGNUP_BEGIN.$SIGNUP_BODY.$SIGNUP_END, TRUE, $signup_shortcodes);
Couldn't be they changed to new array?

Why do I ask? Thank require_once there is the way how to extend template with another plugin. But it fails with signup page because those variables are hardcoded at the beginning of the file:

$SIGNUP_BEGIN = null;
$SIGNUP_BODY = null;
$SIGNUP_END  = null;
$COPPA_TEMPLATE = null;
$COPPA_FAIL = null;

Thanks