css will only change how the browser displays the text, it does not actually change the true state of the text.
think in terms of a fun house mirror, it will make you look short and squat or tall and thin, but that doesn't mean that you _become_ short and squat or tall and thin.
The css is displaying the text as capitol, but in reality, it is still lowercase.
To physically change the case of the text, you could use the PHP strtoupper function to convert the string to upper case:
<?php echo(strtoupper("this is a lower case string")); ?>