导航箭头未显示几张图像

Question

If you go here , and click on the images, then each image has 'next' and 'previous' arrows, but not the first two images. Why is this happening?

<nav class="prev-next">
    <ul>
    <?php
    $next_post = get_next_post();
    $prev_post = get_previous_post();
    $next_post_year = '';
    $previous_post_year = '';
    $previous_post_media = '';
    $next_post_media = '';
    if($next_post) {

    $next_post_id  = ($next_post->ID);
    $years = wp_get_post_terms( $next_post_id, 'years', array( 'fields' => 'ids' ) );
    $medias = wp_get_post_terms( $next_post_id, 'media', array( 'fields' => 'ids' ) );
    $next_post_year = $years[0];
    $next_post_media = $medias[0];

    }
    if($prev_post) {
    $previous_post_id  = ($prev_post->ID);
    $years = wp_get_post_terms( $previous_post_id, 'years', array( 'fields' => 'ids' ) );
    $medias = wp_get_post_terms( $previous_post_id, 'media', array( 'fields' => 'ids' ) );
    $previous_post_year = $years[0];
    $previous_post_media = $medias[0];

    }
    $show_next = false;
    if($next_post_year == $current_post_year && $next_post_media == $current_post_media) {
        $show_next = true;
    }
    $show_prev = false;
    if($previous_post_year == $current_post_year && $previous_post_media == $current_post_media) {
        $show_prev = true;

    }
    ?>
        <?php /*<li class="prev"><?php previous_post('%', '&lsaquo; Previous', 'no'); ?></li>
        <li class="next"><?php next_post('%' , 'Next &rsaquo;', 'no'); ?></li>*/ ?>
        <?php if($show_next) { ?>
        <li class="prev"><?php next_post('%' , '&lsaquo; Previous', 'no'); ?></li>
        <?php } ?>
        <?php if($show_prev) { ?>
        <li class="next"><?php previous_post('%', 'Next &rsaquo;', 'no'); ?></li>
        <?php } ?>      
    </ul>
</nav>

If you look at the single image page's code of the images that are not showing the arrows, you will see that the next and previous <li> are not even being created on those pages.

Answer 1

<li> is not created, because this condition doesn't meet: <?php if($show_next) { ?> . And this condition is about checking if years are matched: if($next_post_year == $current_post_year . Check, if you really need this condition.