A plugin that takes the schema from Yoast SEO and renders it as a hidden microdata div on a page.

schema-microdata.php

<?php
/**
 * Plugin Name: Yoast Schema to Microdata
 * Description: Mirrors Yoast SEO's Schema graph (via Surfaces API) as Microdata inside a hidden <div>, preserving @id as itemid.
 * Version:     0.4.0
 * Author:      Your Name
 * License:     GPL-2.0-or-later
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

if ( ! class_exists( 'JDV_Microdata_Plugin' ) ) {

	final class JDV_Microdata_Plugin {

		/** @var JDV_Microdata_Plugin|null */
		private static $instance = null;

		/** ID used on the wrapper div */
		private $container_id = 'jdv-microdata';

		/**
		 * Singleton.
		 */
		public static function instance() : self {
			if ( null === self::$instance ) {
				self::$instance = new self();
			}
			return self::$instance;
		}

		/**
		 * Wire up hooks.
		 */
		private function __construct() {
			$hook               = apply_filters( 'jdv_microdata_output_hook', 'wp_footer' ); // e.g. 'wp_footer'
			$this->container_id = apply_filters( 'jdv_microdata_container_id', $this->container_id );

			add_action( $hook, [ $this, 'render_hidden_div' ], 99 );
		}

		/**
		 * Output microdata mirror inside a hidden div.
		 */
		public function render_hidden_div() : void {
			if ( is_admin() || ( defined( 'REST_REQUEST' ) && REST_REQUEST ) ) {
				return;
			}
			if ( ! function_exists( 'YoastSEO' ) ) {
				return;
			}

			$meta = YoastSEO()->meta->for_current_page(); // Surfaces API.

			// Expect: $meta->schema as array with @graph and $meta->main_schema_id.
			if ( empty( $meta ) || empty( $meta->schema ) || ! is_array( $meta->schema ) ) {
				return;
			}

			$schema = $meta->schema;
			$graph  = isset( $schema['@graph'] ) ? $schema['@graph'] : ( $schema['graph'] ?? [] );
			if ( empty( $graph ) || ! is_array( $graph ) ) {
				return;
			}

			$index     = $this->index_graph( $graph );
			$main_id   = ! empty( $meta->main_schema_id ) ? $meta->main_schema_id : '';
			$root_node = $main_id && isset( $index[ $main_id ] ) ? $index[ $main_id ] : $this->guess_root( $index );

			if ( empty( $root_node ) ) {
				return;
			}

			$visited = [];
			$html    = $this->render_node( $root_node, $index, $visited );

			if ( $html ) {
				$container_attr = sprintf(
					'id="%s" style="display:none" aria-hidden="true" hidden',
					esc_attr( $this->container_id )
				);

				echo "\n<!-- Yoast Schema → Microdata mirror -->\n";
				echo '<div ' . $container_attr . '>' . $html . "</div>\n";
			}
		}

		/**
		 * Build @id → node index.
		 *
		 * @param array $graph
		 * @return array
		 */
		private function index_graph( array $graph ) : array {
			$index = [];
			foreach ( $graph as $node ) {
				if ( is_array( $node ) && isset( $node['@id'] ) ) {
					$index[ $node['@id'] ] = $node;
				}
			}
			return $index;
		}

		/**
		 * Fallback for a sensible root when main_schema_id is missing.
		 *
		 * @param array $index
		 * @return array|null
		 */
		private function guess_root( array $index ) {
			foreach ( $index as $node ) {
				$type  = $node['@type'] ?? null;
				$types = is_array( $type ) ? $type : ( $type ? [ $type ] : [] );
				if ( in_array( 'WebPage', $types, true ) || in_array( 'Article', $types, true ) ) {
					return $node;
				}
			}
			return $index ? reset( $index ) : null;
		}

		/**
		 * Render a node as a nested itemscope. Avoid loops via $visited.
		 *
		 * @param array|string $node_or_id
		 * @param array        $index
		 * @param array        $visited
		 * @param string|null  $prop_name
		 * @return string
		 */
		private function render_node( $node_or_id, array $index, array &$visited, $prop_name = null ) : string {
			// Resolve references.
			if ( is_string( $node_or_id ) && isset( $index[ $node_or_id ] ) ) {
				$node = $index[ $node_or_id ];
			} elseif ( is_array( $node_or_id ) ) {
				$node = $node_or_id;
			} else {
				return '';
			}

			// Prevent cycles.
			$node_id = $node['@id'] ?? null;
			if ( $node_id ) {
				if ( isset( $visited[ $node_id ] ) ) {
					return '';
				}
				$visited[ $node_id ] = true;
			}

			// Types.
			$types = [];
			if ( isset( $node['@type'] ) ) {
				$types = is_array( $node['@type'] ) ? $node['@type'] : [ $node['@type'] ];
				$types = array_filter( array_map( 'sanitize_text_field', $types ) );
			}

			// Attributes for the wrapper.
			$attrs = [ 'itemscope' => 'itemscope' ];
			if ( ! empty( $types ) ) {
				$attrs['itemtype'] = esc_attr( implode( ' ', array_map(
					function( $t ) { return 'https://schema.org/' . $t; },
					$types
				) ) );
			}
			if ( $prop_name ) {
				$attrs['itemprop'] = esc_attr( $prop_name );
			}

			// New: preserve Yoast @id as itemid, behind a filter.
			$use_itemid = apply_filters( 'jdv_microdata_use_itemid', true, $node, $prop_name );
			if ( $use_itemid && $node_id ) {
				// itemid expects a valid URL or unique string. We pass through Yoast's @id.
				$attrs['itemid'] = esc_url( $node_id );
			}

			$html = '<div ' . $this->attrs( $attrs ) . '>';

			// Properties, skipping JSON-LD control keys.
			foreach ( $node as $prop => $value ) {
				if ( 0 === strpos( (string) $prop, '@' ) ) {
					continue;
				}
				// Preserve Schema.org camelCase: no lowercasing. Strip only unsafe chars.
				$prop_out = $this->normalize_prop( (string) $prop );
				if ( $prop_out === '' ) {
					continue;
				}
				$html .= $this->render_property( $prop_out, $value, $index, $visited );
			}

			$html .= '</div>';

			return $html;
		}

		/**
		 * Render a single property value.
		 *
		 * @param string $prop
		 * @param mixed  $value
		 * @param array  $index
		 * @param array  $visited
		 * @return string
		 */
		private function render_property( string $prop, $value, array $index, array &$visited ) : string {
			// Scalar.
			if ( is_scalar( $value ) ) {
				$content = is_bool( $value ) ? ( $value ? 'true' : 'false' ) : (string) $value;

				// If scalar matches a known @id, expand as nested itemscope.
				if ( is_string( $value ) && isset( $index[ $value ] ) ) {
					return $this->render_node( $index[ $value ], $index, $visited, $prop );
				}

				return '<meta itemprop="' . esc_attr( $prop ) . '" content="' . esc_attr( $content ) . '" />';
			}

			// Lists.
			if ( is_array( $value ) && array_values( $value ) === $value ) {
				$html = '';
				foreach ( $value as $entry ) {
					$html .= $this->render_property( $prop, $entry, $index, $visited );
				}
				return $html;
			}

			// Objects.
			if ( is_array( $value ) ) {
				// Pure reference object { "@id": "..." }.
				if ( isset( $value['@id'] ) && count( $value ) === 1 && isset( $index[ $value['@id'] ] ) ) {
					return $this->render_node( $index[ $value['@id'] ], $index, $visited, $prop );
				}
				// Inline nested object.
				return $this->render_node( $value, $index, $visited, $prop );
			}

			return '';
		}

		/**
		 * Format attributes, handling itemscope.
		 *
		 * @param array $attrs
		 * @return string
		 */
		private function attrs( array $attrs ) : string {
			$parts = [];
			foreach ( $attrs as $k => $v ) {
				if ( $v === 'itemscope' ) {
					$parts[] = 'itemscope';
				} else {
					$parts[] = $k . '="' . $v . '"';
				}
			}
			return implode( ' ', $parts );
		}

		/**
		 * Preserve camelCase while removing unsafe characters for an HTML attribute value.
		 * Allows letters, digits, underscore, dash, colon, and dot.
		 */
		private function normalize_prop( string $prop ) : string {
			return preg_replace( '/[^A-Za-z0-9_:\.\-]/', '', $prop );
		}
	}

	// Bootstrap.
	add_action( 'plugins_loaded', [ 'JDV_Microdata_Plugin', 'instance' ] );
}

/**
 * Developer notes:
 * - Filters:
 *   - 'jdv_microdata_output_hook' to move output, default 'wp_footer'.
 *   - 'jdv_microdata_container_id' to change the hidden div ID, default 'jdv-microdata'.
 *   - 'jdv_microdata_use_itemid' to enable or disable itemid, default true.
 * - We preserve Schema.org camelCase for itemprop names, no forced lowercasing.
 * - Each itemscope with an @id now includes itemid="<same value>" which keeps entity identity aligned with Yoast’s graph.
 */