Add custom Field after content

Hello everyone,
I have this code in function.php:
https://gist.github.com/anonymous/3d8e3f3d878ca10dfdd2
I would like to add a new field that should be displayed after the content.
After the last line of code I have added this function:

add_action('__after_content','my_acf',100);
function my_acf() {
if ( ! is_single() )
    return;
<?php

$posts = get_field('fumetti');

if( $posts ): ?>
	<div class="box-tipo"><div class="box-campo">Relazione Fumetti:</div><div class="box-risultato"><ul class="box-voci">
	<?php foreach( $posts as $p ): // variable must NOT be called $post (IMPORTANT) ?>

	    	<li><a href="<?php echo get_permalink( $p->ID ); ?>"><?php echo get_the_title( $p->ID ); ?></a></li>

	<?php endforeach; ?></ul></div></div>

<?php endif; ?>
}

Unfortunately it seems not to work. Where did I go wrong?