stephencottontail on "Customizing Footer"

You need to close off the PHP tags before you can put HTML tags or you'll get syntax errors. Try this:

<?php if ( get_theme_mod( 'copyright_textbox' ) ) : ?>
    <div class="site-info">
        <?php echo get_theme_mod( 'copyright_textbox' ); ?>
    </div><!-- .site-info -->
<?php else : ?>
    <div class="site-info">
       ... default WordPress footer...
    </div><!-- .site-info -->
<?php endif; ?>