Get and store Posts from another site using the WP REST API

newsletter_posts.php

<?php

function prefix_get_newsletter_posts() {

    // Get the cached data if available, if not fetch it using the WP REST API
    if ( false === ( $newsletter_posts_data = get_transient( 'prefix_newsletter_posts' ) ) ) {

        // Tag ID for the Posts you want from the Newsletter site.
        $tag_id = 12;

        $response = wp_remote_get( 'http://newsletter.site.com/wp-json/wp/v2/posts?tags=' . $tag_id );
        
        if ( is_wp_error( $response ) ) {

            // HTTP Request Failed
            return false;

        } else {

            $body = wp_remote_retrieve_body( $response );
            
            $newsletter_posts = json_decode( $body );

            if ( isset( $newsletter_posts[0]->link ) ) {

                // Store the data for 30mins
                // Edit this if you want to change how long the data is cached for.
                set_transient( 'prefix_newsletter_posts', $body, 30 * MINUTE_IN_SECONDS );

            }

            return $newsletter_posts;

        }

    }

    $newsletter_posts = json_decode( $newsletter_posts_data );

    return $newsletter_posts;

}

function the_newsletter_posts() {

    $newsletter_posts = get_newsletter_posts(); ?>
    
    if ( $posts ) {
    
    echo '<ul>';

    foreach ( $newsletter_posts as $news_post ) { ?>

      <li><a href="<?php echo esc_url( $news_post->link ); ?>" title="<?php echo esc_attr( $news_post->title->rendered ); ?>"><?php echo esc_html( $news_post->title->rendered ); ?></a></li>
                                 
    <?php }
    
    echo '</ul>';

    }

}