Resource proxy with search and replace functionaltiy

resource_proxy.php

<?php
        /* 
          Resource proxy with search and replace functionality on resources (e.g. css, js) in order to get around CORS restrictions.
                  
          https://hubbounce.services.example.co.uk/?resource_url=https://example.com/wp-content/themes/example-training/dist/css/main.min.css&amp;search=url(../fonts/&amp;replace=url(https://cdn2.hubspot.net/hubfs/4668366/fonts/
        */

        $resource_url = $_GET['resource_url'];
        $search = $_GET['search'];
        $replace = $_GET['replace'];

        $returned_data = '';

        // check if url before
        if(filter_var($resource_url, FILTER_VALIDATE_URL) === FALSE) {
                header("HTTP/1.0 404 Not Found");
                echo "Not found.\n";
                echo $resource_url;
                die();
        } else {
                $returned_data = file_get_contents($resource_url);
                header("Content-type: text/css; charset: UTF-8");
                echo str_replace($search, $replace, $returned_data);
        }

?>