How can I effectively implement HTTP caching with PHP for dynamic websites with static content?

Integrating HTTP Caching with PHP

In situations where page content remains primarily static but surrounded by a dynamic template, leveraging HTTP cache headers can optimize page delivery. Here's a simplified guide to implementing effective caching using PHP:

Essential Headers for Caching

To enable caching, consider sending the following headers:

• Content-Type: Specify the MIME type and character set of the response.
• Vary: Accept: Indicate that the response can vary based on the client's Accept header, controlling content negotiation.
• Cache-Control: Set directives like private_no_expire to allow private caching but prevent revalidation. Alternatively, use public to allow shared caching.
• Last-Modified: Provide the date and time when the content was last modified.
• ETag: Assign a unique identifier to the content, which clients can use to determine if the cached version is still current.

Conditional Requests and Handling

Process incoming conditional requests using if-modified-since and if-none-match:

• if-modified-since: Compare the request header with the Last-Modified header. If the cached version is still valid, return a 304 (Not Modified) status code.
• if-none-match: Validate against the ETag header. If the cached version matches, return a 304.

Determining Cache Validity

When generating the ETag, consider using a checksum or a combination of factors like user ID, language, and timestamp. For longer-lasting static content, set a longer expiration.

Example Implementation

<code class="php">$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 &amp;&amp; $if_none_match == $etag) || (!$if_none_match)) &amp;&amp; 
($if_modified_since &amp;&amp; $if_modified_since == $tsstring))
{
header('HTTP/1.1 304 Not Modified');
exit();
}
else
{
header("Last-Modified: $tsstring");
header("ETag: \"{$etag}\"");
}</code>

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