Looking at the Html helper file in the CakePHP library located at lib/Cake/View/Helper/HtmlHelper.php, check out line 427, looks like the .css extension is set automatically unless there are two slashes in the CSS file name. I GUESS that is to catch external resources?
From the aforementioned library file:
if (strpos($path, '//') !== false) {
$url = $path;
} else {
$url = $this->assetUrl($path, $options + array('pathPrefix' => CSS_URL, 'ext' => '.css'));
if (Configure::read('Asset.filter.css')) {
$pos = strpos($url, CSS_URL);
if ($pos !== false) {
$url = substr($url, 0, $pos) . 'ccss/' . substr($url, $pos + strlen(CSS_URL));
}
}
}
So this..
echo $this->Html->css('http://whatever.com/css/dynamic-stylesheet.php');
Would render the dynamic-stylesheet.php file rather than defaulting to 'dynamic-stylesheet.css.
Not sure that is what this was intended for but maybe that file will help you.