WordPress [esi] Shortcode for doing Edge Side Includes (ESI)

wordpress-esi-shortcode.php

<?php

/**
 * Edge Side Includes (ESI) Shortcode
 */
function esi_shortcode($attrs){
	$attrs = shortcode_atts(array(
		'src' => '',
		'ttl' => '1h',
		'dca' => 'none',
		'onerror' => 'continue'
	), $attrs);
	
	// Serialize $attrs into HTML attributes
	$html_attrs = array();
	foreach($attrs as $name => $value){
		$html_attrs[] = $name . '="' . esc_attr($value) . '"';
	}
	
	// Yes, we return the esi shortcode back with the updated attributes because
	// WordPress will strip out <esi:include> at this point. The content filter
	// below will do the final replacement just in time.
	return sprintf('[esi %s ]', join(' ', $html_attrs));
}
add_shortcode( 'esi', 'esi_shortcode' );


/**
 * Since <esi:include> tags don't make it through the WordPress content filter
 * when output by shortcode, do the replacement at the end.
 */
function esi_shortcode_content_filter( $content ){
	return preg_replace_callback( '/\[esi(.+?)\]/', 'esi_shortcode_content_filter_callback', $content );
}
add_filter( 'the_content', 'esi_shortcode_content_filter', 100 );


/**
 * This is because Akamai b0rks on &amp; appearing in esi:include attributes
 */
function esi_shortcode_content_filter_callback( $matches ){
	$attrs = str_replace( '&amp;', '&', $matches[1] );
	return "<esi:include $attrs />";
}

Just for clarification:

[esi src="http://example.com ttl="3h"]

will produce...

<esi:include src="http://example.com ttl="3h" dca="none" onerror="continue" />