gistfile1.txt

<?php
/*
Plugin Name: Block URLs
Plugin URI: https://dustinrue.com
Description: Block select domains from being accessed by WordPress. Rather than blocking all domains, this plugin allows you to block just specific domains instead.
Author: Dustin Rue
Version: 0.0.1
*/

/*
Usage: Similar to WP_ACCESSIBLE_HOSTS, define WP_BLOCKED_HOSTS with a comma separated list of domains

define( 'WP_BLOCKED_HOSTS', 'api.wordpress.org,*.themeisle.com' );
*/

function block_urls( $preempt, $parsed_args, $uri ) {

    if ( ! defined( 'WP_BLOCKED_HOSTS' ) ) {
      return false;
    }

    $check = parse_url( $uri );
    if ( ! $check ) {
      return false;
    }

    static $blocked_hosts = null;
    static $wildcard_regex   = array();
    if ( null === $blocked_hosts ) {
        $blocked_hosts = preg_split( '|,\s*|', WP_BLOCKED_HOSTS );
        if ( false !== strpos( WP_BLOCKED_HOSTS, '*' ) ) {
          $wildcard_regex = array();
          foreach ( $blocked_hosts as $host ) {
            $wildcard_regex[] = str_replace( '\*', '.+', preg_quote( $host, '/' ) );
          }
          $wildcard_regex = '/^(' . implode( '|', $wildcard_regex ) . ')$/i';
        }
    }

    if ( ! empty( $wildcard_regex ) ) {
      $results = preg_match( $wildcard_regex, $check['host'] );
      if ($results > 0) {
        error_log(sprintf("Blocking %s://%s%s", $check['scheme'], $check['host'], $check['path']));
      } else {
        error_log(sprintf("Allowing %s://%s%s", $check['scheme'], $check['host'], $check['path']));
      }

      return $results > 0;
    } else {
      $results = in_array( $check['host'], $blocked_hosts, true ); // Inverse logic, if it's in the array, then block it.

      if ($results) {
        error_log(sprintf("Blocking %s://%s%s", $check['scheme'], $check['host'], $check['path']));
      } else {
        error_log(sprintf("Allowing %s://%s%s", $check['scheme'], $check['host'], $check['path']));
      }
      return $results;
    }
}

add_filter('pre_http_request', 'block_urls', 10, 3);