Can’t add secondary_title to a php page

<ul class="posts">
  <?php foreach ($posts as $single ) { // iterates the posts ?>
	<li>
	  <?php echo mysql2date('d', $single->post_date) ?> <a href="<?php echo get_permalink($single->ID); ?>"><?php echo get_the_title($single->ID); ?></a>  
	  <?php echo get_secondary_title(); ?> 
	</li>
  <?php } // ends foreach $posts ?>
</ul> <!-- ul.posts -->

Hello, I have added the above code in an index-page.php file that is in my theme folder and I use it as a template page.

But I can’t make secondary_title to show.