いいね機能（jsを使った実装）

functions.php

// いいね機能
add_action('wp_ajax_nopriv_get_like_count', 'get_like_count');
add_action('wp_ajax_get_like_count', 'get_like_count');
function get_like_count() {
    if (!isset($_POST['post_id'])) {
        wp_die('Post ID is missing');
    }

    $post_id = intval($_POST['post_id']);
    $likes = get_post_meta($post_id, 'likes_count', true);
    $likes = $likes ? (int)$likes : 0;

    wp_send_json(array('likes' => $likes));
}
add_action('wp_head', 'my_ajaxurl');
function my_ajaxurl() {
    echo '<script type="text/javascript">
        var ajaxurl = "' . admin_url('admin-ajax.php') . '";
    </script>';
}

add_action('wp_ajax_nopriv_process_like_cookie', 'process_like_cookie');
add_action('wp_ajax_process_like_cookie', 'process_like_cookie');

function process_like_cookie() {
    if (!isset($_POST['post_id'])) {
        wp_die('Post ID is missing');
    }

    $post_id = intval($_POST['post_id']);
    $likes = get_post_meta($post_id, 'likes_count', true);
    $likes = $likes ? (int)$likes : 0;

    error_log("Initial likes for post {$post_id}: {$likes}");

    // Check if cookie exists
    if (isset($_COOKIE['liked_' . $post_id])) {
        // Cookie exists, user has already liked this post, so we "unlike" it
        if ($likes > 0) {
            $likes--;
            update_post_meta($post_id, 'likes_count', $likes);
            setcookie('liked_' . $post_id, '', time() - 3600, '/'); // Remove the cookie
            error_log("User unliked post {$post_id}, new likes: {$likes}");
        } else {
            error_log("User tried to unlike post {$post_id}, but likes are already 0");
        }
    } else {
        // No cookie, user hasn't liked this post, so we "like" it
        $likes++;
        update_post_meta($post_id, 'likes_count', $likes);
        setcookie('liked_' . $post_id, 'yes', time() + 86400 * 30, '/'); // 30 days
        error_log("User liked post {$post_id}, new likes: {$likes}");
    }

    echo $likes;
    wp_die();
}

header.php

< link の記載が連続してあるので、その後ろに以下を追加。

<link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.1/css/all.min.css" rel="stylesheet">	

同じように <script の記載が連続してあるので、その後ろに以下を追加。

<script>
jQuery(document).ready(function($) {
    function updateLikeButton(post_id) {
        var $likeIcon = $('.like-button[data-postid="' + post_id + '"]').find('i');
        if (getCookie('liked_' + post_id)) {
            $likeIcon.removeClass('far').addClass('fas');
        } else {
            $likeIcon.removeClass('fas').addClass('far');
        }
    }

    function getCookie(name) {
        var value = "; " + document.cookie;
        var parts = value.split("; " + name + "=");
        if (parts.length === 2) return parts.pop().split(";").shift();
    }

    $('.like-button').each(function() {
        var post_id = $(this).data('postid');
        updateLikeButton(post_id);
    });

    $('.like-button').on('click', function(e) {
        e.preventDefault();
        var $likeButton = $(this);
        var post_id = $likeButton.data('postid');
        var $likeCount = $('#like-count-' + post_id);
        var currentCountText = $likeCount.text();
        var is999Plus = currentCountText === '999+';
        var currentCount = parseInt(currentCountText);
        var buttonOffset = $likeButton.offset();
        var buttonHeight = $likeButton.outerHeight();
		var wasLiked = getCookie('liked_' + post_id);

        if ($likeButton.data('processing')) {
            return;
        }

        $likeButton.data('processing', true);

        $.ajax({
            type: "POST",
            url: ajaxurl,
            data: {
                action: "process_like_cookie",
                post_id: post_id,
            },
            success: function(response) {
                var newCount = parseInt(response);
                updateLikeButton(post_id);

                if (newCount > 999) {
                    $likeCount.text('999+');
                } else {
                    $likeCount.text(newCount);
                }

                if (newCount > currentCount && newCount > 999) {
                    $likeCount.animate({
                        fontSize: '1.8rem'
                    }, 500, function() {
                        $(this).animate({
                            fontSize: '1.6rem'
                        }, 500);
                    });
                }
				if (!wasLiked) {
					$('#like-message').css({
						'position': 'absolute',
						'top': (buttonOffset.top + buttonHeight + 5) + 'px',
						'left': buttonOffset.left + 'px',
						'display': 'block'
					}).fadeIn(500).delay(2000).fadeOut(500, function() {
						$(this).css('display', 'none');
					});
				}
                $likeButton.data('processing', false);
            }
        });
    });
});
</script>

style.css

.like-button i {
    color: #FF2F40; 
}

.like-button {
	    font-size: 24px;
    background: none; /* 背景色を透明に */
    border: none; /* 枠線を削除 */
    padding-left: 20px;
    padding-right: 0px;
    margin: 10px; /* マージンを削除 */
    color: inherit; /* ボタンのテキスト色を親要素から継承 */
}

.like-button:hover {
    cursor: pointer; /* マウスカーソルをポインターに変更 */
    opacity: 0.7; /* ホバー時にボタンを少し透明にする */
}

/* いいねされた時のメッセージ */
#like-message {
    display: none;
    position: absolute; /* 固定位置から絶対位置に変更 */
    background-color: #ff8793; /* ピンク色 */
    color: white;
    padding: 10px 20px;
    border-radius: 5px;
    z-index: 1000;
    font-size: 16px;
}

functions.phpで強制読み込みを実施

footer.php

htmlの閉じタグの直前に以下を追加
（ブログによってカスタマイズする場合はメッセージを変える）

<div id="like-message" style="display: none;">ありがとうございます！</html>

single.php

<?php the_content(); ?>

のすぐ下に

<?php
echo '<button class="like-button" data-postid="'.get_the_ID().'">';
echo '<i class="far fa-heart"></i>';
echo '</button>';
?>

<span id="like-count-<?php echo get_the_ID(); ?>"></span>
<script>
var postId = <?php echo get_the_ID(); ?>;
var initialLikeCount = <?php echo get_post_meta(get_the_ID(), 'likes_count', true) ?: 0; ?>;
</script>
<script src="<?php echo get_template_directory_uri(); ?>/like-count.js?v=<?php echo filemtime(get_template_directory() . '/like-count.js'); ?>"></script>	

以上