Reduce Bandwidth Usage by Supporting If-Modified-Since in PHP

Sunday, 30 September 2007

By default, pages generated with PHP are not cached by browsers or proxies, as they are generated anew every time the page is loaded by the server. If you have repeat visitors to your website, or even many visitors that use the same proxy, this means that a lot of bandwidth is wasted transferring content that hasn't changed since last time. By adding appropriate code to your PHP pages, you can allow your pages to be cached, and reduce the required bandwidth.

As Bruce Eckel points out in RSS: The Wrong Solution to a Broken Internet, this is a particular problem for RSS feeds — feed readers are often overly enthusiastic in their checking rate, and given the tendency of bloggers to provide full feeds this can lead to a lot of wasted bandwidth. By using the code from this article in your feed-generating code you can save yourself a whole lot of bandwidth.

Caching and HTTP headers

Whenever a page is requested by a browser, the server response includes a Last-Modified header in the response which indicates the last modification time. For static pages, this is the last modification time of the file, but for dynamic pages it typically defaults to the time the page was requested. Whenever a page is requested that has been seen before, browsers or proxies generally take the Last-Modified time from the cached version and populate an If-Modified-Since request header with it. If the page has not changed since then, the server should respond with a 304 response code to indicate that the cached version is still valid, rather than sending the page content again.

To handle this correctly for PHP pages requires two things:

• Identifying the last modification time for the page, and
• Checking the request headers for the If-Modified-Since.

Timestamps

There are two components to the last modification time: the date of the data used to generate the page, and the date of the script itself. Both are equally important, as we want the page to be updated when the data changes, and if the script has been changed the generated page may be different (for example, the layout could be different). My PHP code incorporates both by defaulting the modification time of the script, and allowing the user to pass in the data modification time, which is used if it is more recent than the script. The last modification time is then used to generate a Last-Modified header, and returned to the caller. Here is the function that adds the Last-Modified header. It uses both getlastmod() and filemtime(__FILE__) to determine the script modification time, on the assumption that this function is in a file included from the main script, and we want to detect changes to either.

function setLastModified($last_modified=NULL)
{
    $page_modified=getlastmod();
    
    if(empty($last_modified) || ($last_modified < $page_modified))
    {
        $last_modified=$page_modified;
    }
    $header_modified=filemtime(__FILE__);
    if($header_modified > $last_modified)
    {
        $last_modified=$header_modified;
    }
    header('Last-Modified: ' . date("r",$last_modified));
    return $last_modified;
}

Handling If-Modified-Since

If the If-Modified-Since request header is present, then it can be parsed to get a timestamp that can be compared against the modification time. If the modification time is older than the request time, a 304 response can be returned instead of generating the page.

In PHP, the HTTP request headers are generally stored in the $_SERVER superglobal with a name starting with HTTP_ based on the header name. For our purposes, we need the HTTP_IF_MODIFIED_SINCE entry, which corresponds to the If-Modified-Since header. We can check for this with array_key_exists, and parse the date with strtotime. There's a slight complication in that old browsers used to add additional data to this header, separated with a semicolon, so we need to strip that out (using preg_replace) before parsing. If the header is present, and the specified date is more recent than the last-modified time, we can just return the 304 response code and quit — no further output required. Here is the function that handles this:

function exitIfNotModifiedSince($last_modified)
{
    if(array_key_exists("HTTP_IF_MODIFIED_SINCE",$_SERVER))
    {
        $if_modified_since=strtotime(preg_replace('/;.*$/','',$_SERVER["HTTP_IF_MODIFIED_SINCE"]));
        if($if_modified_since >= $last_modified)
        {
            header("HTTP/1.0 304 Not Modified");
            exit();
        }
    }
}

Putting it all together

Using the two functions together is really simple:

     exitIfNotModifiedSince(setLastModified()); // for pages with no data-dependency
     exitIfNotModifiedSince(setLastModified($data_modification_time)); // for data-dependent pages

Of course, you can use the functions separately if that better suits your needs.

Posted by Anthony Williams
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