Adjusting caption below single post image

Override The Image Caption With Image Title And Description

Here’s a way to override the caption with the image title and image description, with a demo plugin.

Let’s assume we have uploaded this image:

Then we insert it into the editor with the following:

  <a href="https://example.tld/2017/03/01/hello-world/espresso/"    
       rel="attachment wp-att-123">
    <img src="https://example.tld/wp-content/uploads/2017/03/espresso-300x180.jpg" 
         alt="" width="300" height="180" class="size-medium wp-image-123" />        
   </a> espresso caption

Before:

Without our plugin activated it will display as:

After:

With the plugin activated it displays:

Hope it helps!

Demo plugin:

We use the filters img_caption_shortcode and shortcode_atts_caption.

Here we wrap it into a class to pass on the attachment id between the filter callbacks.

<?php
/**
 * Plugin Name: Modified Image Caption
 * Description: Override The Image Caption With Image Title And Description
 * Plugin URI:  http://wordpress.stackexchange.com/a/258612/26350
 */

namespace WPSE\Q258586;

add_action( 'init', [ new Caption, 'init' ] );

class Caption
{
    private $attachment_id ;

    public function init()
    {
        add_filter( 'img_caption_shortcode',    [ $this, 'shortcode' ], 999, 3 );
        add_filter( 'shortcode_atts_caption',   [ $this, 'atts' ],      999, 3 );
    }
    public function shortcode( $output, $attr, $content )
    {
        if( isset( $attr['id'] ) && preg_match( '/attachment_\d+/i', $attr['id' ] ) )
            $this->attachment_id = str_replace( 'attachment_', '', $attr['id'] );

        return $output;
    }

    public function atts( $atts, $pair )
    {
        if( ! $this->attachment_id  )
            return $atts;

        $img = get_post( $this->attachment_id );

        if( is_a( $img, '\WP_Post' ) )
        {
            $atts['caption'] = sprintf( 
                '<span class="title-css">%s</span>
                 <br><span class="description-css">%s</span>',
                esc_html( $img->post_title ),
                esc_html( $img->post_content )
            );
        }

        $this->id = null;

        return $atts;
    }
}