Function to delete all post metas of a specific key

[alphaoide]

We need a function which takes a meta key as an argument and deletes all post metas with that specified meta key.

It's useful for a plugin which store some internal metas and need to delete those in its uninstall routine.

[markjaquith]

API ++

[ryan]

(In [5976]) delete_post_meta_by_key() from Mark Jaquith. fixes #4178