WordPress Multiblog: Combine WP-API feeds from different websites and cache the output

functions.php

<?php
/**
 * WordPress "Multiblog"
 *
 * Combine multiple WP-API feeds sorted by date and cache the output via Transients API
 */

// Cache transients in database: https://codex.wordpress.org/Transients_API
$transient = 'wp_api_remote_posts_cache;
$content = get_transient( $transient ); // Try to get cached data

if ( empty( $content ) ) :
	$response_mainsite = wp_remote_get( get_rest_url( null, 'wp/v2/posts ) );

	$urls = array();
	$urls[] = 'https://secondwebsite.com/wp-json/wp/v2/posts';
	$urls[] = 'https://thirdwebsite.com/wp-json/wp/v2/posts';
	
	if ( ! is_wp_error( $response_mainsite ) ) :
		$data_mainsite = json_decode( wp_remote_retrieve_body( $response_mainsite ) );
	endif;

	if ( ! empty( $urls ) ) :
		$data_mergedsites = array();

		foreach ( $urls as $url ) {
			$response_site = wp_remote_get( $url );

			if ( ! is_wp_error( $response_site ) ) :
				$data_site = json_decode( wp_remote_retrieve_body( $response_site ) );
				//print_r( $data_site );

				$data_mergedsites = array_merge( $data_mergedsites, $data_site );
			endif;
		}

		// Merge data from main site with data from other sites
		$recent_posts = array_merge( $data_mainsite, $data_mergedsites );

		// Sort merged data by date
		usort( $recent_posts, function ( $a, $b ) {
			return strtotime( $b->date ) - strtotime( $a->date );
		} );
	else :
		$recent_posts = $data_mainsite;
	endif;

	//print_r( $recent_posts );

	if ( ! empty( $recent_posts ) ) :
		$content .= '<ul class="recentposts">';
			$i = 0;

			foreach ( $recent_posts as $recent_post ) :
				++$i;

				// Limit to 10 posts
				if ( $i <= 10 ) :
					$content .= '<li>';
						$content .= '<h4 class="widget-title"><a href="' . esc_url( $recent_post->link ) . '" rel="bookmark">' . esc_html( $recent_post->title->rendered ) . '</a></h4>';
					$content .= '</li>';
				endif;
			endforeach;
		$content .= '</ul>';

		set_transient( $transient, $content, 12 * HOUR_IN_SECONDS ); // Cache data for 1/2 day
	endif;
endif;

echo $content;