Docs: Block Variations - Add PHP strategy to register block variations using get_block_type_variations hook

[juanmaguitar]

Block Editor Handbook > Reference Guides > Block API Reference > Variations is lacking a description of how to add variations to a core block from PHP side

An approach that could be recommended from the docs is the use of the get_block_type_variations hook to add any custom variations to blocks on register-time.

This approach is the one used in Gutenberg