Manage easily HTTP caching

routines.php

<?php
function LastModified(Closure $when, Closure $noncached) {
    $rmtime = $when();
    if (array_key_exists('HTTP_IF_MODIFIED_SINCE', $_SERVER)) {
        $lmtime = date_create($_SERVER['HTTP_IF_MODIFIED_SINCE']);
        if (FALSE !== $lmtime && $lmtime >= $rmtime) {
            header('HTTP/1.1 304 Not Modified', TRUE, 304);
            exit;
        }
    }
    header('Cache-Control: max-age=86400, public');
    header('Last-Modified: ' . $rmtime->format(DateTime::RFC1123));
    $noncached();
}

function ETag(Closure $when, Closure $noncached) {
    $etag = md5($when());
    if (array_key_exists('HTTP_IF_NONE_MATCH', $_SERVER) && $_SERVER['HTTP_IF_NONE_MATCH'] == $etag) {
        header('HTTP/1.1 304 Not Modified', TRUE, 304);
        exit;
    }
    header('Cache-Control: max-age=86400, public');
    header('ETag: ' . $etag);
    $noncached();
}

sample.php

<?php
$dbh = new PDO(...);

// articles#show
LastModified(
    function () use ($dbh) {
        if (!isset($_GET['id']) || FALSE === ($id = filter_var($_GET['id'], FILTER_VALIDATE_INT))) {
            header('HTTP/1.1 400 Bad Request', TRUE, 400);
            exit;
        }
        $stmt = $dbh->prepare('SELECT updated_at FROM articles WHERE id = ?');
        $stmt->execute(array($_GET['id']));
        if (FALSE === ($updated_at = $stmt->fetchColumn()) {
            header('HTTP/1.1 404 Not Found', TRUE, 404);
            exit;
        }
        return new DateTime($updated_at);
    },
    function () use ($dbh) {
        $stmt = $dbh->prepare('SELECT * FROM articles WHERE id = ?');
        $stmt->execute(array($_GET['id']));
        $article = $stmt->fetch();

        printf('<h1>%s</h1>', htmlspecialchars($article['title'], ENT_QUOTES, 'UTF-8'));
        $sd = new Sundown($article['text']);
        echo $sd->to_html();
    }
);

// articles#index
ETag(
    function () use ($dbh) {
        return implode(',', $dbh->query('SELECT id FROM articles ORDER BY updated_at DESC')->fetchAll(PDO::FETCH_COLUMN, 0));
    },
    function () use ($dbh) {
        echo '<h1>...</h1>';
        foreach ($dbh->query('SELECT * FROM articles ORDER BY updated_at DESC') as $art) {
            echo '<div class="...">';
            printf('<h2>%s</h2>', htmlspecialchars($article['title'], ENT_QUOTES, 'UTF-8'));
            printf('<p>%s</p>', htmlspecialchars($article['summary'], ENT_QUOTES, 'UTF-8'));
            echo '</div>';
        }
    }
);