Using HTTP Cache Headers to Optimize PHP Website Performance

Despite the availability of advanced cache systems and frameworks, sometimes it's necessary to implement basic HTTP cache headers to improve website performance. Particularly for PHP 5.1.0 websites, which lack certain cache capabilities, headers play a vital role.

Effective Cache Headers for PHP Websites

To enable effective caching, consider the following headers:

• Last-Modified: Sets the last modified date of the document, which browsers use to determine if the content has changed since their last visit.
• ETag: Specifies a unique identifier for the document, allowing browsers to check if they already have the latest version.
• Expires: Sets an absolute expiration time for the document, instructing browsers not to cache it beyond that time.
• Cache-Control: Controls how and for how long browsers should cache the document. It can be set to "public," allowing anyone to cache the document, or "private," restricting caching to individual users. You might want to consider using "private_no_expire" if you need longer caching lengths without allowing public caching.

Implementing Conditional Requests

In addition to setting cache headers, it's essential to handle conditional requests, such as "If-Modified-Since" and "If-None-Match." These requests allow browsers to check if the document has changed since a specific date or if the ETag has changed since their last visit.

If the content hasn't changed, you can return a "304 Not Modified" status code, indicating that the browser can continue using the cached version. This efficient approach minimizes unnecessary downloads and speeds up website loading.

The following PHP code sample demonstrates how to handle conditional requests:

$timestamp = /* PHP script to generate a unique timestamp */;
$tsstring = gmdate('D, d M Y H:i:s ', $timestamp) . 'GMT';
$etag = $language . $timestamp;

$if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : false;
$if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? $_SERVER['HTTP_IF_NONE_MATCH'] : false;
if ((($if_none_match && $if_none_match == $etag) || (!$if_none_match)) &&
    ($if_modified_since && $if_modified_since == $tsstring))
{
    header('HTTP/1.1 304 Not Modified');
    exit();
}
else
{
    header("Last-Modified: $tsstring");
    header("ETag: \"{$etag}\"");
}

By implementing cache headers and handling conditional requests effectively, you can significantly improve the performance and user experience of your PHP website.