PHP

Quines in web scripting languages are interesting.  When the quine is viewed in a web browser, it appears the same regardless of whether the web server is configured to run the script or just serve the file.  (Try it!):

<?php header('Content-Type: text/plain');
function q($s) { echo "$s q('", addcslashes($s, "\\'"), "');"; }
function r($s) { echo "r('$s'); $s"; } q('<?php header(\'Content-Type: text/plain\');
function q($s) { echo "$s q(\'", addcslashes($s, "\\\\\'"), "\');"; }
function r($s) { echo "r(\'$s\'); $s"; }');r('?>'); ?>