Убедитесь, что перенаправление является последним шагом. Примерно так:
function my_module_form {
$form['#action'] = 'some.external.site';
# Store the redirect in a value element, maybe we need some data
# which are passed to the form, like a user ID, node ID etc.
# So lets store the link in a value element
$form['redirect_link'] = array(
'#type' => 'value',
'#value' => url('some.redirect.page'),
);
}
function my_module_form_validate ($form, &$form_state) {
# Do some validation stuff...
}
function my_module_form_submit($form, &$form_state) {
# show some confirmation message...
drupal_set_message(t('Successfully sent your data into space.'));
# And finally the redirect...
# The 'redirect_link' value was passed internally, without being sent
# to the browser so we can safely use it.
$form_state['redirect'] = $form_state['values']['redirect_link']
}