proxy ByWater's Coce cover image cache server (for use on an HTTPS domain)

cover.php

<?php
// this proxies ByWater Solutions' "COCE" cover image service
// which does not work over HTTPS, so fill in our libraries.cca.edu
// server as our COCE server & it intercepts requests, sending along
// data from ByWater's COCE server

// we're sending JS
header( 'Content-Type:application/javascript; charset=utf-8' );

// requests look like
// coce.bywatersolutions.com/cover?id=0231081596,0816614024&provider=aws,gb,ol
// &callback=jQuery17209698950295937665_1468611400913&_=1468611401160
$id = $_GET['id'];
$provider = $_GET['provider'];

function get_coce ($id, $provider) {
    // return only JSON, no need for callback wrapper
    return file_get_contents(
        'http://coce.bywatersolutions.com/cover?id=' . $id . '&provider=' . $provider
    );
}

$json = get_coce($id, $provider);

// we sometimes get an empty response on first try, attempt again
if ($json = '{}') {
    $json = get_coce($id, $provider);
}

// if we have callpack param, wrap it around JSON, otherwise send just JSON
if (isset($_GET['callback'])) {
    echo $_GET['callback'] . '(' . $json . ')';
} else {
    echo $json;
}