semantic figure and figcaption elements for captioned images in WordPress – insert into functions.php

caption_shortcode

function caption_shortcode($val, $attr, $content = null) {
	extract(shortcode_atts(array('id'=> '','align'=> 'aligncenter','width'=> '','caption' => ''), $attr));
	
	if ( 1 > (int) $width || empty($caption) )return $val;
	$capid = '';
	
	if ( $id ) {
		$id = esc_attr($id);
		$capid = 'id="figcaption_'. $id . '" ';
		$id = 'id="' . $id . '" aria-labelledby="figcaption_' . $id . '" ';
	}

	return '<figure ' . $id . 'class="post-figure figure-' . esc_attr($align) . '" style="width: '. (int) $width . 'px">' 
		. do_shortcode( $content ) . '<figcaption ' . $capid . 'class="post-figcaption">' . $caption . '</figcaption></figure>';
}	

add_filter('img_caption_shortcode', 'caption_shortcode', 10, 3);