How should I handle container form elements?

Question

I have added some custom fields to the register user form and I'm discovered that they are sent back, packaged in a form element of type "container". My code needs to handle all kind of different custom register fields.

How should I parse these objects and extract the fields from them? Should I look into the arrays and what can I expect to find there?

I can't seem to find any good documentation on these objects.

Edit:

A regular fields received through user registration might look like this:

name
... (Array, 10 elements)
#type (String, 9 characters ) textfield
#title (String, 10 characters ) Username
#maxlength (Integer) 60
#required (Boolean) TRUE
#attributes (Array, 1 element)
class (Array, 1 element)
0 (String, 8 characters ) username
#default_value (String, 0 characters )
#access (Boolean) TRUE
#weight (Integer) -10
#fb_registration_field (String, 4 characters ) name

A form container looks e.g. like this:

field_birtyday
... (Array, 6 elements)
#type (String, 9 characters ) container
#attributes (Array, 1 element)
#weight (String, 1 characters ) 7
#tree (Boolean) TRUE
#language (String, 3 characters ) und
und (Array, 13 elements)
0 (Array, 12 elements)
#entity_type (String, 4 characters ) user
#bundle (String, 4 characters ) user
#field_name (String, 14 characters ) field_birtyday
#language (String, 3 characters ) und
#field_parents (Array, 0 elements)
#columns (Array, 2 elements)
0 (String, 5 characters ) value
1 (String, 6 characters ) format
#title (String, 8 characters ) Birthday
#description (String, 0 characters )
#required (Boolean) FALSE
#delta (Integer) 0
#weight (Integer) 0
value (Array, 16 elements)
#entity_type (String, 4 characters ) user
#bundle (String, 4 characters ) user
#field_name (String, 14 characters ) field_birtyday
#language (String, 3 characters ) und
#field_parents (Array, 0 elements)
#columns (Array, 2 elements)
0 (String, 5 characters ) value
1 (String, 6 characters ) format
#title (String, 8 characters ) Birthday
#description (String, 0 characters )
#required (Boolean) FALSE
#delta (Integer) 0
#weight (Integer) 0
#type (String, 9 characters ) textfield
#default_value (NULL)
#size (String, 2 characters ) 60
#maxlength (String, 3 characters ) 255
#attributes (Array, 1 element)
#theme (String, 25 characters ) field_multiple_value_form | (Callback) field_multiple_value_form();
#field_name (String, 14 characters ) field_birtyday
#cardinality (String, 1 characters ) 1
#title (String, 8 characters ) Birthday
#required (Integer) 0
#description (String, 0 characters )
#prefix (String, 42 characters ) <div id="field-birtyday-add-more-wrapper">
#suffix (String, 6 characters ) </div>
#max_delta (Integer) 0
#after_build (Array, 1 element)
0 (String, 30 characters ) field_form_element_after_build | (Callback) field_form_element_after_build();
#language (String, 3 characters ) und
#field_parents (Array, 0 elements)

Answer 1

You should use $form_state['values'] to handle submitted user data, not $form.

Answer 2

There is no good documentation on that form because it is dynamic, any module can implement hook_form_alter() and completely turn it upside down. In your example, the name is added directly by user.module while field_birthday is a field and fields are built in a way that allows them to be displayed multiple times if necessary. That's why they are a container and only inside them is the actual form element.

Assuming you want to parse the actual form, and not the response, you have to go through the form recursively. The function element_children() can help you with that. It returns the keys of all form elements below the one you passed in to that function.