Converts post categories into CSS classes

post-cats-to-css-classes.php

<?php
function categories_to_classes()
{
	$out  = '';
	$cats = get_the_category();

	foreach ( $cats as $cat_object )
	{
		$out .= $cat_object->slug . ' ';
	}

	return trim($out);
}

// Usage
?>
<div class="<?php
print categories_to_classes();
?>"> content </div>