This is really a general WP question and not specific to ACF. I built a plugin that does this using shortcodes https://github.com/Hube2/blunt-snippets. There are other plugins available that work the same way, like this one: https://wordpress.org/plugins/allow-php-in-posts-and-pages/
As far as entering PHP directly into a WYSIWYG editor, I don’t know how you’d do that and to be honest I don’t think it’s a good idea.
