Submission handler for the user registration form doesn't work
In hook_form_alter use foloowing code


function MODULENAME_form_alter(&$form, $form_state, $form_id) {
if($form_id == 'user_register_form'){
    $form['#submit'][] = 'module_custom_form_submit';
   }
}


after that use 



function module_custom_form_submit($form, &$form_state) {
    // Do whatever you want.
}