PHP allows you to change the header of a file or page so that it renders differently in the browser. This is VERY powerful if you know how to use it properly.
With these headers you can make anything have server side logic. Any static format (css, xml, jpg, etc) can easily be dynamic with the help of php and the header() function.
If you would like to see a simple example, check out the Dynamic CSS article.
I cannot remember where I got this list from, I've always had it in a saved text file. I've only added a couple headers on my own. If you know the owner of the original list, feel free to let me know so I can give the appropriate credits.
// fix 404 pages:
header('HTTP/1.1 200 OK');
// set 404 header:
header('HTTP/1.1 404 Not Found');
// set Moved Permanently header (good for redrictions)
// use with location header
header('HTTP/1.1 301 Moved Permanently');
// Internal server error
header('HTTP/1.1 500 Internal Server Error');
// redirect to a new location:
header('Location: http://www.example.org/');
// redrict with delay:
header('Refresh: 10; url=http://www.example.org/');
print 'You will be redirected in 10 seconds';
// you could also use the HTML syntax:
// <meta http-equiv="refresh" content="10;http://www.example.org/ />
// override X-Powered-By: PHP:
header('X-Powered-By: PHP/4.4.0');
header('X-Powered-By: Brain/0.6b');
// content language (en = English)
header('Content-language: en');
// last modified (good for caching)
$time = time() - 60; // or filemtime($fn), etc
header('Last-Modified: '.gmdate('D, d M Y H:i:s', $time).' GMT');
// header for telling the browser that the content
// did not get changed
header('HTTP/1.1 304 Not Modified');
// set content length (good for caching):
header('Content-Length: 1234');
// Headers for an download:
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="example.zip"');
header('Content-Transfer-Encoding: binary');
// load the file to send:
readfile('example.zip');
// Disable caching of the current document:
header('Cache-Control: no-cache, no-store, max-age=0, must-revalidate');
header('Expires: Mon, 26 Jul 1997 05:00:00 GMT'); // Date in the past
header('Pragma: no-cache');
// set content type:
header('Content-Type: text/html; charset=iso-8859-1');
header('Content-Type: text/html; charset=utf-8');
header('Content-Type: text/plain'); // plain text file
header('Content-Type: text/css'); // CSS file
header('Content-Type: image/jpeg'); // JPG picture
header('Content-Type: application/zip'); // ZIP file
header('Content-Type: application/pdf'); // PDF file
header('Content-Type: audio/mpeg'); // Audio MPEG (MP3,...) file
header('Content-Type: application/x-shockwave-flash'); // Flash animation
// show sign in box
header('HTTP/1.1 401 Unauthorized');
header('WWW-Authenticate: Basic realm="Top Secret"');
print 'Text that will be displayed if the user hits cancel or ';
print 'enters wrong login data';