Caching HTTP responses when they are dynamically created by PHP

by

Serving huge or many auxiliary files with PHP is not exactly what it’s made for.

Instead, look at X-accel for nginx, X-Sendfile for Lighttpd or mod_xsendfile for Apache.

The initial request gets handled by PHP, but once the download file has been determined it sets a few headers to indicate that the server should handle the file sending, after which the PHP process is freed up to serve something else.

You can then use the web server to configure the caching for you.

Static generated content

If your content is generated from PHP and particularly expensive to create, you could write the output to a local file and apply the above method again.

If you can’t write to a local file or don’t want to, you can use HTTP response headers to control caching:

Expires: <absolute date in the future>
Cache-Control: public, max-age=<relative time in seconds since request>

This will cause clients to cache the page contents until it expires or when a user forces a page reload (e.g. press F5).

Dynamic generated content

For dynamic content you want the browser to ping you every time, but only send the page contents if there’s something new. You can accomplish this by setting a few other response headers:

ETag: <hash of the contents>
Last-Modified: <absolute date of last contents change>

When the browser pings your script again, they will add the following request headers respectively:

If-None-Match: <hash of the contents that you sent last time>
If-Modified-Since: <absolute date of last contents change>

The ETag is mostly used to reduce network traffic as in some cases, to know the contents hash, you first have to calculate it.

The Last-Modified is the easiest to apply if you have local file caches (files have a modification date). A simple condition makes it work:

if (!file_exists('cache.txt') || 
    filemtime('cache.txt') > strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
    // update cache file and send back contents as usual (+ cache headers)
} else {
    header('HTTP/1.0 304 Not modified');
}

If you can’t do file caches, you can still use ETag to determine whether the contents have changed meanwhile.

More Related Contents:

  1. HTTP authentication logout via PHP
  2. Easy way to test a URL for 404 in PHP?
  3. How do I read any request header in PHP
  4. Return a PHP page as an image
  5. HTTP Headers for File Downloads
  6. How to prevent my site page to be loaded via 3rd party site frame of iFrame
  7. How to clear browser cache with php?
  8. What is causing “Unable to allocate memory for pool” in PHP?
  9. Check if PHP-page is accessed from an iOS device
  10. Getting HTTP code in PHP using curl
  11. Symfony: Clear doctrine cache
  12. How to get the browser to cache images, with PHP?
  13. file_get_contents() give me 403 Forbidden
  14. Preventing Caching of CSS Files
  15. Request headers bag is missing Authorization header in Symfony 2?
  16. Compile a referenced LESS file into CSS with PHP automatically
  17. How to cache in Symfony 2?
  18. How to use the CSV MIME-type?
  19. failed to open stream: HTTP wrapper does not support writeable connections
  20. How can I set a cookie and then redirect in PHP?
  21. PHP cURL how to add the User Agent value OR overcome the Servers blocking cURL requests?
  22. How to properly use Bearer tokens?
  23. header(“Content-type: text/css”); is working in Firefox and Chrome, but in Internet Explorer 9 it shows up as ‘text/html’
  24. Can I serve MP3 files with PHP?
  25. Laravel 5 Clear Views Cache
  26. Is header(‘Content-Type:text/plain’); necessary at all?
  27. using header() to rewrite filename in URL for dynamic pdf
  28. prevent browser back button cache
  29. Reload the page on hitting back button
  30. Downloading a file with a different name to the stored name

Leave a Comment