One-Click “Set as Featured Image” Button for Any Image Block (Classic & Custom Themes)

// Add "Set as Featured Image" button below images in the Classic Editor
add_filter('the_content', function($content) {
    if (!is_admin() || !current_user_can('edit_posts')) return $content;

    // Add button after each <img> tag in the editor
    $content = preg_replace_callback(
        '/<img[^>]+src="([^"]+)"[^>]*>/i',
        function($matches) {
            $img = $matches[0];
            $src = esc_url($matches[1]);
            $button = '<button class="set-featured-image-btn" style="display:block;margin:6px 0 18px 0;padding:4px 10px;font-size:12px;cursor:pointer;" data-img="' . $src . '">Set as Featured Image</button>';
            return $img . $button;
        },
        $content
    );
    return $content;
});

// Enqueue the JS to handle the button click
add_action('admin_footer', function() {
    global $pagenow;
    if ($pagenow !== 'post.php' && $pagenow !== 'post-new.php') return;
    ?>
    <script>
    document.addEventListener('DOMContentLoaded', function() {
        document.querySelectorAll('.set-featured-image-btn').forEach(function(btn) {
            btn.addEventListener('click', function(e) {
                e.preventDefault();
                var imgUrl = btn.getAttribute('data-img');
                // AJAX call to set featured image
                var data = new FormData();
                data.append('action', 'set_featured_image_by_url');
                data.append('img_url', imgUrl);
                data.append('post_id', document.getElementById('post_ID').value);
                fetch(ajaxurl, { method: 'POST', credentials: 'same-origin', body: data })
                .then(response => response.json())
                .then(res => {
                    if(res.success) {
                        btn.innerText = 'Featured Image Set!';
                        setTimeout(() => { btn.innerText = 'Set as Featured Image'; }, 2000);
                    } else {
                        alert('Failed to set featured image.');
                    }
                });
            });
        });
    });
    </script>
    <?php
});

// AJAX handler to set featured image by URL
add_action('wp_ajax_set_featured_image_by_url', function() {
    if (!current_user_can('edit_posts')) wp_send_json_error();

    $post_id = intval($_POST['post_id']);
    $img_url = esc_url_raw($_POST['img_url']);

    // Try to get attachment ID by URL
    $attachment_id = attachment_url_to_postid($img_url);
    if ($attachment_id && get_post($attachment_id)) {
        set_post_thumbnail($post_id, $attachment_id);
        wp_send_json_success();
    } else {
        wp_send_json_error();
    }
});