All functions must be implemented, see their individual documentation about what exactly they should do.
Configuration, if necessary, should be done using the MidCOM configuration system, prefixing all values with 'auth_frontend_$name_', e.g. 'auth_frontend_form_cssclass'.
ArrayA simple associative array with the two indexes 'username' and 'password' holding the information read by the driver or null if no information could be read.
This content you print is assumed to work within an HTML DIV element, so you should usually stick to a simple form, which should also by styleable using CSS alone.
You should use HTTP POST to submit the form data to the page you originated from.
If you really need to redirect to some external page, ensure that you send the user back to the original location unharmed. Be aware that this type of operation is strongly discouraged.
You MAY send HTTP Authentication headers if your auth driver uses them and stop execution immediately afterwards. (2DO: How to treat sent content (it is in the output buffer) at this point?)