キャプションショートコードのfigureにaria-labelledbyを、figcaptionにそのidを付ける

functions.php

<?php

//キャプションにaria-labelledbyを付ける
add_shortcode('caption', 'my_img_caption_shortcode');
function my_img_caption_shortcode( $attr, $content = null ) {
  // New-style shortcode with the caption inside the shortcode with the link and image tags.
  if ( ! isset( $attr['caption'] ) ) {
    if ( preg_match( '#((?:<a [^>]+>\s*)?<img [^>]+>(?:\s*</a>)?)(.*)#is', $content, $matches ) ) {
      $content = $matches[1];
      $attr['caption'] = trim( $matches[2] );
    }
  } elseif ( strpos( $attr['caption'], '<' ) !== false ) {
    $attr['caption'] = wp_kses( $attr['caption'], 'post' );
  }

  $output = apply_filters( 'img_caption_shortcode', '', $attr, $content );
  if ( $output != '' )
    return $output;

  $atts = shortcode_atts( array(
    'id'    => '',
    'align'   => 'alignnone',
    'width'   => '',
    'caption' => '',
    'class'   => '',
  ), $attr, 'caption' );

  $atts['width'] = (int) $atts['width'];
  if ( $atts['width'] < 1 || empty( $atts['caption'] ) )
    return $content;

  if ( ! empty( $atts['id'] ) )
    //$aria_idを追加
    $aria_id = trim( 'caption-' . $atts['id'] );
    $atts['id'] = 'id="' . esc_attr( sanitize_html_class( $atts['id'] ) ) . '" ';

  $class = trim( 'wp-caption ' . $atts['align'] . ' ' . $atts['class'] );

  $html5 = current_theme_supports( 'html5', 'caption' );
  // HTML5 captions never added the extra 10px to the image width
  $width = $html5 ? $atts['width'] : ( 10 + $atts['width'] );

  $caption_width = apply_filters( 'img_caption_shortcode_width', $width, $atts, $content );

  $style = '';
  if ( $caption_width )
    $style = 'style="width: ' . (int) $caption_width . 'px" ';

  $html = '';
  if ( $html5 ) {
    //aria-labelledbyに$aria_idを入れる
    $html = '<figure ' . $atts['id'] . $style . 'class="' . esc_attr( $class ) .'"' . 'aria-labelledby="' . esc_attr( $aria_id ) .'">'
      //figcaptionのidに$aria_idを入れる
    . do_shortcode( $content ) . '<figcaption ' . 'id="' . esc_attr( $aria_id ) .'"' . 'class="wp-caption-text">' . $atts['caption'] . '</figcaption></figure>';
  } else {
    $html = '<div ' . $atts['id'] . $style . 'class="' . esc_attr( $class ) . '">'
    . do_shortcode( $content ) . '<p class="wp-caption-text">' . $atts['caption'] . '</p></div>';
  }

  return $html;
}