JSON编码字符集问题

I am having some character encoding issues with my json_encode function:

I have a javascript variable which stores a string of text for me:

<script type='text/javascript'>
 postQuote = <?php echo json_encode($post['post_text']); ?>
</script>

$post['post_text'] is taken from the MySQL database, where it shows correctly. If I just print out $post['post_text'] there will be no problem in the encoding.

Now, the string can example be:

You're doing great today? That's perfect.

Now the postQuote will output:

You're doing great today? That's perfect

How can I solve this?