single.php

<?php

/**
 * The Template for displaying all single posts.
 *
 * @package Gridspace
 * @since Gridspace 1.0
 */

get_header();

$html = <<<BANNER
	<div align="center">
		<a href="https://www.facebook.com/pages/abc">
			<img src="http://shuttermuse.com/assets/images/Facebook_newsletter-banner.jpg" />
		</a>
	</div>
BANNER;

add_filter('the_content', function ($content) use ($html)
{
	return $content . $html;
});

?>

<section id="primary">

	<?php if ( have_posts() ) while ( have_posts() ) : the_post(); ?>

	<?php // gridspace_content_nav( 'nav-above' ); ?>

	<?php  get_template_part( 'content', get_post_format() ); ?>

	<?php gridspace_content_nav( 'nav-below' ); ?>

	<?php comments_template( '', true ); ?>

	<?php endwhile; // end of the loop. ?>

</section><!-- #primary -->

<?php get_footer(); ?>