Naming Custom Fields

What is the general naming convention for custom fields?  I'm having trouble with the API and I'm wondering if it has to do with the custom fields that I have set up.

I have custom fields such as:

State/Province [state,fallback=] text

When using the API in PHP would the code look like this:

$state = new SubscriberCustomField();
$state->Key = '[state]';
$state->Value = $user5;

- OR -

$state = new SubscriberCustomField();
$state->Key = '[State/Province]';
$state->Value = $user5;

I'm eagerly awaiting a response. :-)

Thanks!
Sean

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