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Run actions when a post, page, or custom post type is about to be trashed. The post ID is passed to the action hook.

Example:

Parameters

$postid

(integer) (required) The ID of the post, page, or custom post type being deleted.

Default: None

 add_action( 'trash_post', 'function_to_run_on_post_trash' );

 function function_to_run_on_post_trash( $post_id ) {
   // trash a related post or fire notice
   
 }