How to implement this cache control policy in PHP?

Question

Serve the new content only if the file has changed since last visit.

How do I implement that?

UPDATE

Sorry not to mention it earlier,but the requested resource is web page which is requested directly,not an image.

Answer 1

You can use a trick borrowed from rails and append the last file modification time to the include:

$fileName = 'image.jpg';
$httpLink = $fileName . '?' . filemtime( $fileName );
echo '<img src="', $fileName, '" alt="blah" />';

This will output a link like

<img src="image.jpg?1002412" alt="blah" />   

Then when the file changes, the query string will also change and the browser will request the "new" file i.e.

<img src="image.jpg?1003622" alt="blah" />

Alternatively you could keep a local log of file revisions and read the revision number from a database rather than the filesystem, which may be marginally faster (and save filesystem reads, although it's not significant difference - dependant on db vs web server load).