Handling If-modified-since header in a PHP-script

Question

I have a PHP script which is called with an ?img= parameter. The value for that parameter is an (urlencoded) URL of an image. My script checks, if that image is already stored at my server. If not - it downloads it. After that it optionally resizes the image and sends it to STDOUT, i.e. back to the requesting browser, prepended with Content-Type and Last-modified headers: ``` Connection:close Content-Type:image/jpeg Date:Fri, 01 Jun 2012 08:28:30 GMT Last-Modified:Fri, 01 Jun 2012 08:02:44 GMT Server:Apache/2.2.15 (CentOS) Transfer-Encoding:chunked X-Powered-By:PHP/5.3.3 ``` This is needed to workaround some crossdomain issues and works well for me since over a year: However I'd like to add functionality to handle the incoming If-Modified-since header - to send a Not Modified 304 response. My questions are: 1) Is that even possible in PHP, when run in Apache? 2) How to handle (i.e. parse and produce) the dates best in PHP here? Bonus question) How to add a Content-Length header for th…

Accepted Answer

This is definitely possible in PHP!

When the browser checks if there were modifications, it sends an `If-Modified-Since` header; in PHP this value would be set inside `$_SERVER['HTTP_IF_MODIFIED_SINCE']`.

To decode the date/time value (encoded using rfc822 I believe), you can just use `strtotime()`, so:

```
if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) && 
    strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= filemtime($localFileName))
{
    header('HTTP/1.0 304 Not Modified');
    exit;
}
```

Explanation: if the `If-Modified-Since` header is sent by the browser AND the date/time is at least the modified date of the file you're serving, you write the "304 Not Modified" header and stop.

Otherwise, the script continues as per normal.

Handling If-modified-since header in a PHP-script

Solution

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