Proper handling of IfModifiedSince with PHP :: oyoy.eu search engine tools

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Usage:

Create a new module with the code below.
Add the following snippet to all your pages that have dated content:
```
<?php
include("handle304.php");
setModifiedDate( strtotime('11 September 2003') ); // use the date of the content, NOT time() (just an example)
?>
```
Instead of the strtotime() function you can directly specify the date and time of your content (in UTC).

Another idea would be to use the date/time of the script file itself, if your scripts contain the actual content (and it is not being pulled from a database):
```
<?php
include("handle304.php");
clearstatcache(); 
setModifiedDate( filemtime(__FILE__) ); // use the date of the file itself
?>
```
(Thanks, Craig from cass-hacks.com!) The call to clearstatcache() helps to make sure that the real date is used and not one that was just cached.
Test with IfModifiedSince tester
Done!

I'm open to all corrections, additions or neat tricks. Feel free to contact me via the contact-link in the footer.

The code for the module handle304.php:

<?php
function setModifiedDate($contentDate) {
    $ifModifiedSince = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? stripslashes($_SERVER['HTTP_IF_MODIFIED_SINCE']) : false;
    if ($ifModifiedSince && strtotime($ifModifiedSince) >= $contentDate) {
        header('HTTP/1.0 304 Not Modified');
        die; // stop processing
    }
    $lastModified = gmdate('D, d M Y H:i:s', $contentDate) . ' GMT';
    header('Last-Modified: ' . $lastModified);
}
?>

Some comments from webado:

As for how I determine the last modified date of a page made up of different included modules, I do a bit of juggling.

Let's say my page.php is like this:

<?php
$module1="module1.inc";
$module2="module2.inc";
$module1="module3.inc";
 
include("setlastmod.php");
include("template.php");
?>

And let's say the script template.php is an html document with places where I have:

<!DOCTYPE ..>
<html>
<head>
...
...
</head>
<body>
..
..
include($module1);
...
...
...
include($module2);
...
...
include($module3);
...
</body>
</html>

The script setlastmod.php painstakingly (;) ) goes through all of those modules and determines it's last modified date, and I then keeps the latest as the overall last modified date for the page where they are getting used.

The script setlastmod.php is like this:

<?php
clearstatcache();
$lm1 = filemtime($module1);
 
clearstatcache();
$lm2 = filemtime($module2);
 
clearstatcache();
$lm3 = filemtime($module3);
$lastmod = max($lm1,$lm2,$lm3);
$last_modified = gmdate('D, d M Y H:i:s', $lastmod).' GMT';
header("Last-Modified: $last_modified");
?>

I am currently setting the header for the last modified date according to this method.

Handling IfModifiedSince requests with PHP