<?php
/**
  * Passes through image from cache folder to render.
  * @param string $fileName
  * @param string $fileExtension
  */
 public static function renderImageFromCacheFolder($fileName, $fileExtension){
     $filePath = self::getImagesCacheFolderPath(). $fileName . ".". $fileExtension;
     // get the last modification of the image
     $lastModified = filemtime($filePath);

    //Modify last access time by using touch, so when we run cleanCache we can delete all images not accessed in the last 30 days
    // ensure only access time is changed, not modification time, do this by setting new modification time to the
    // previous one, and access time to current time()
    touch($filePath, date('U', $lastModified), time());

    // get a unique hash of this image
    // this is called an etag
    $etagFile = md5_file($filePath);

    // get the HTTP_IF_MODIFIED_SINCE header if set
    // imagine this as the browser saying to your server:
    //   "Don't mind sending me the image if it wasn't modified since this date, I already have it then"
    $ifModifiedSince=(isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false);

    // get the HTTP_IF_NONE_MATCH header if set (etag: unique file hash)
    // imagine this as the browser saying to your server:
    //  "Don't mind sending me the image if it has this etag, if file's etag on the server is the same
    //      etag that I have, then we have the same image, Don't send it to me, I already have it"
    $etagHeader=(isset($_SERVER['HTTP_IF_NONE_MATCH']) ? trim($_SERVER['HTTP_IF_NONE_MATCH']) : false);

    //set last-modified header
    header("Last-Modified: ".gmdate("r", $lastModified));
    //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 file wasn't modified, or etag still the same
    // then we don't need to send it again
    // we check using "or" because here we use md5_file to get the etag. then *even* if the modified date changes
    // if md5_file stilll returns the same result, the file didn't change at all
    if (($ifModifiedSince && @strtotime($ifModifiedSince)==$lastModified) || $etagHeader == $etagFile)
    {
           header("HTTP/1.1 304 Not Modified");
           exit;
    }

    // at this point we need to send the image to the browser because it's not in browser cache
    header("Content-Type: image/$fileExtension");
    header('Content-Length: ' . filesize($filePath));

    echo file_get_contents($filePath);
}