if ( ! function_exists( 'mm_post_nav' ) ) :
/**
 * Displays navigation to next/previous post when applicable.
 *
 * @since 1.0
 *
 * @return void
  */
function mm_post_nav() {
global $post;

// Don't print empty markup if there's nowhere to navigate.
$previous = ( is_attachment() ) ? get_post( $post->post_parent ) : get_adjacent_post( false, '', true );
$next     = get_adjacent_post( false, '', false );
$prevpost = get_previous_post(true);
$prevThumbnail = get_the_post_thumbnail($prevpost->ID, array(44,44) );
$nextpost = get_next_post(true);
$nextThumbnail = get_the_post_thumbnail($nextpost->ID, array(44,44) );
if ( ! $next && ! $previous )
    return;
?>
<nav class="mm-post-nav" role="navigation">
    <h1 class="screen-reader-text"><?php _e( 'Post navigation', 'mm-cube' ); ?></h1>
    <div class="mm-single-nav">
        <div class="mm-prev-link">

            <?php previous_post_link( '%link', _x( '<i class="icon-chevron-left"></i><span class="mm-prev-post-title"> %title</span>', 'Previous post link', 'mm-cube' ) ); ?>
        </div>
        <div class="mm-next-link">

            <?php next_post_link( '%link', _x( '<span class="mm-next-post-title">%title</span><i class="icon-chevron-right"></i>' , 'Next post link', 'mm-cube' ) ); ?>

        </div>
    </div><!-- .nav-links -->
    <div class="clear"></div>
</nav><!-- .navigation -->
<?php
}
endif;

string(0) ""

$prevpost = get_previous_post(true);
if (!empty( $prevpost )) {
    $prevThumbnail = get_the_post_thumbnail($prevpost->ID, array(44,44) );
}
$nextpost = get_next_post(true);
if (!empty( $nextpost )) {
    $nextThumbnail = get_the_post_thumbnail($nextpost->ID, array(44,44) );
}

previous_post_link( $format, $link, $in_same_cat = false, $excluded_terms = '', $taxonomy = 'category' );

previous_post_link( '%link', _x( '<i class="icon-chevron-left"></i><span class="mm-prev-post-title">%title</span>', FALSE, 'mm-cube' ) );