A simple script to help answer this SO question: https://stackoverflow.com/questions/48013106/fetch-and-cache-a-json-file/48021592#48021592

json-load-and-cacher.php

<?php
class JsonLoadAndCacher {
    public $url;
    public $hash;
    //set cache directory here
    private $fileLocation = 'd:/temp/';

    public function LoadJson($url) {
        $hash = md5($url);
        //set your cached offline file as variable for ease of use
        $cachedFile = $this->fileLocation . $hash;
        //check if file exists
        if (file_exists($cachedFile)) {
            echo "cache exists\n";
            //check if file age is within 24 hours
            if(time() - filemtime($filename) > (24 * 3600)) {
                echo "cache is within 24 hours\n";
                return file_get_contents($cachedFile);
            }
        }

        echo "cache doesn't exists or is older than 24 hours\n";
        //cache doesn't exist of is older than 24 hours
        //download it
        $jsonFile = file_get_contents($url);
        // Save content into cache
        file_put_contents($cachedFile, $jsonFile);
        //return downloaded content as result
        return $jsonFile;
    }
}

test.php

<?php
require_once('json-load-and-cacher.php');

$MyJson = new JsonLoadAndCacher;
$content = $MyJson->LoadJson('http://mysafeinfo.com/api/data?list=englishmonarchs&format=json');