functions.php displays in header

Hi, this is my first time coding a wp theme. I have problem with functions.php content, since it displays in my site’s header. I have absolutely no idea why this is happening.
content of my functions.php is this:

// Enable post thumbnails
add_theme_support('post-thumbnails');
set_post_thumbnail_size(600, 420, true);

Could this be caused by wrong markup?

Thank you for any help!