On my test case, the page’ title is “Privacy”. The straightforward approach

strtolower(the_title())

also returns “Privacy” — still with a capital P. I also tried

mb_strtolower(the_title())

and

mb_strtolower(the_title(), 'utf8')

with similar results.

Not sure if it matters, but my wp-config.php has

define('WPLANG', '');
define('DB_CHARSET', 'utf8');

lowercasing the title

If I’m understanding you correctly, you should be doing:

strtolower(get_the_title());

or

print strtolower(get_the_title());

if you want to display it. Below is an explanation as to why.

the_title() vs. get_the_title()

The function the_title() prints the current post’s title unless you pass false as its third argument. Unless you call it like:

$title = the_title('', '', false);

The title will be printed, and the $title variable won’t contain anything. This matters because calling strtolower() on an empty variable doesn’t do very much.

You want to use get_the_title() function in most cases where you’re looking to fill a variable with the content posts title.

Note, however, that if you’re not currently in a loop, you’ll need to pass a post ID to get_the_title(). In almost all cases when on a single post or page you can do this by using:

get_the_title($post->ID);

as the $post variable should be in the global scope.