Advanced Custom Fields Show PHP in Text Areas

Pay for the ACF Pro version to enable flexible fields (which is basically the function I wanted to have) or use an alternative WP plugin that has the feature for free, such as Carbon Fields or Piklist.

If you want to display PHP in the textboxes, although creating a security vulnerability, you could use Shortcodes. For instance, to display a field in your WYSIWYG editor, use the following code: [acf field="{$field_name}"] and in order to display it, you need to use <?php echo do_shortcode(get_field('field_name')); ?>.