readme.md

A function allow you to check your page whether it's been modified or not since it was last displayed.

It can also give you a "304" status.

This snippet is by

Intelligent PHP Cache Control, CHRIS COYIER

in

https://css-tricks.com/snippets/php/intelligent-php-cache-control/

NotModifiedCheck.php

<?php

function notModifiedCheck()
{
    //get the last-modified-date of this very file
    $lastModified=filemtime(__FILE__);
    //get a unique hash of this file (etag)
    $etagFile = md5_file(__FILE__);
    //get the HTTP_IF_MODIFIED_SINCE header if set
    $ifModifiedSince=(isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false);
    //get the HTTP_IF_NONE_MATCH header if set (etag: unique file hash)
    $etagHeader=(isset($_SERVER['HTTP_IF_NONE_MATCH']) ? trim($_SERVER['HTTP_IF_NONE_MATCH']) : false);

    //set last-modified header
    header("Last-Modified: ".gmdate("D, d M Y H:i:s", $lastModified)." GMT");
    //set etag-header
    header("Etag: $etagFile");
    //make sure caching is turned on
    header('Cache-Control: public');

    //check if page has changed. If not, send 304 and exit
    if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE'])==$lastModified || $etagHeader == $etagFile)
    {
           header("HTTP/1.1 304 Not Modified");
           exit;
    }

    //your normal code
    echo "This page was last modified: ".date("Y.m.d H:i:s",time());
}