Settings API – getting hidden input / submit button’s name

Using $_POST is too low level. Simply give it the same name as your other options. When you options are displayed they should have the names of the form: my_settings[a_particular_option]. For instance:

<input name="my_settings[some_input_option]"/>

Then for your hidden input and submit button:

<input type="hiddden" name="my_settings[foo]" value="bar"/>
<input name="my_settings[SubmitButton]" type="submit" class="button-primary" value="<?php _e('Save Changes'); ?>"/>

In your validation array

my_validation($input) {
   $foo = $input['foo'];
   $submit = $input['SubmitButton'];
}