cache.php

<?php

function cache(string $content) {
    $filemtime = filemtime(__FILE__);
    $length = strlen($content);
    $etag = '"'.dechex($filemtime).'-'.dechex($length).'"';
    $modified = gmdate("D, d M Y H:i:s", $filemtime)." GMT";
    if ($_SERVER['HTTP_IF_NONE_MATCH'] === $etag || $_SERVER['HTTP_IF_MODIFIED_SINCE'] === $modified) {
        http_response_code(304);
        exit;
    }
    header("Content-Length: $length");
    header("ETag: $etag");
    header("Last-Modified: $modified");
    echo $content;
    exit;
}