I have registered a custom block with ACFs acf_register_block. I have set a render_callback to load a php template but would like to access any extra CSS classes which the user may have set in the admin.
Does WordPress provide a global like $block (similar to $post) or would I need to raise a question on the ACF support forums?
Note that I don’t have ACF PRO 5.8 which comes with the Block features, but I hope this answer helps.
If you look at the example here, you can retrieve the additional/custom CSS classes using props.attributes.className; i.e. the classes are saved as an attribute named className.
So with that in mind and based on acf_register_block() (which as of writing, it redirects to acf_register_block_type()), your render_callback function would receive a $block array (the block settings and attributes) as the first argument, and therefore you should be able to retrieve the additional CSS class(es) via $block['className'] as shown here — see the example under “Registering a block with callback“.
