api-meta-endpoint.php

<?php
/**
 * Retrieve a meta for a post.
 *
 * @uses get_post_custom()
 *
 * @param int $id Post ID
 * @param string $key
 * @return array Post meta fields and values
 */
public function get_post_meta( $id, $context = 'view' ) {
	$id = (int) $id;

	if ( empty( $id ) ) {
		return new WP_Error( 'json_post_invalid_id', __( 'Invalid post ID.' ), array( 'status' => 404 ) );
	}

	$post = get_post( $id, ARRAY_A );
	$post_meta = (array) get_post_custom( $id );

	if ( empty( $post['ID'] ) ) {
		return new WP_Error( 'json_post_invalid_id', __( 'Invalid post ID.' ), array( 'status' => 404 ) );
	}
	if ( empty( $post_meta ) ) {
		return new WP_Error( 'json_post_invalid_meta', __( 'Invalid post_meta.' ), array( 'status' => 404 ) );
	}
	if ( ! $this->check_read_permission( $post ) ) {
		return new WP_Error( 'json_user_cannot_read', __( 'Sorry, you cannot read this post.' ), array( 'status' => 401 ) );
	}

	// Link headers (see RFC 5988)

	$response = new WP_JSON_Response();
	$response->header( 'Last-Modified', mysql2date( 'D, d M Y H:i:s', $post['post_modified_gmt'] ) . 'GMT' );

	$response->link_header( 'alternate',  get_permalink( $id ), array( 'type' => 'text/html' ) );

	$response->set_data( $post_meta );
	return $response;
}