Shortcode to insert a link to undo the action when someone unsubscribed

MailPoet: Shortcode to undo unsubscribe

<?php
/**
 * function to return an undo unsbscribe string for MailPoet newsletters
 * you could place it in the functions.php of your theme
 * @return string
 */
function mpoet_get_undo_unsubscribe(){
    if(class_exists('WYSIJA') && !empty($_REQUEST['wysija-key'])){

        $undo_paramsurl = array(
            'wysija-page'=>1,
            'controller'=>'confirm',
            'action'=>'undounsubscribe',
            'wysija-key'=>$_REQUEST['wysija-key']
            );

        $model_config = WYSIJA::get('config','model');
        $link_undo_unsubscribe = WYSIJA::get_permalink($model_config->getValue('confirmation_page'),$undo_paramsurl);


        $undo_unsubscribe = str_replace(
               array('[link]','[/link]'),
               array('<a href="'.$link_undo_unsubscribe.'">','</a>'),
               '<p><b>'.__('You made a mistake? [link]Undo unsubscribe[/link].',WYSIJA)).'</b></p>';

        return $undo_unsubscribe;
    }
    return '';
}

// will replace the shortcode [mailpoet_undo_unsubscribe]
add_shortcode('mailpoet_undo_unsubscribe', 'mpoet_get_undo_unsubscribe');

// echo the link directly in your theme if you like
echo mpoet_get_undo_unsubscribe();