do_action( 'save_post', int $post_ID, WP_Post $post, bool $update )

More Information

save_post is an action triggered whenever a post or page is created or updated, which could be from an import, post/page edit form, xmlrpc, or post by email. The data for the post is stored in $_POST, $_GET or the global $post_data, depending on how the post was edited. For example, quick edits use $_POST.

Since this action is triggered right after the post has been saved, you can easily access this post object by using get_post($post_id);.

NOTE: As of WP 3.7, an alternative action has been introduced, which is called for specific post types: save_post_{post_type}. Hooking to this action prevents your callback to be unnecessarily triggered.

Avoiding infinite loops

If you are calling a function such as wp_update_post that includes the save_post hook, your hooked function will create an infinite loop. To avoid this, unhook your function before calling the function you need, then re-hook it afterward.

// this function makes all posts in the default category private

function set_private_categories($post_id) {
	// If this is a revision, get real post ID
	if ( $parent_id = wp_is_post_revision( $post_id ) ) 
		$post_id = $parent_id;

	// Get default category ID from options
	$defaultcat = get_option( 'default_category' );

	// Check if this post is in default category
	if ( in_category( $defaultcat, $post_id ) ) {
		// unhook this function so it doesn't loop infinitely
		remove_action( 'save_post', 'set_private_categories' );

		// update the post, which calls save_post again
		wp_update_post( array( 'ID' => $post_id, 'post_status' => 'private' ) );

		// re-hook this function
		add_action( 'save_post', 'set_private_categories' );
	}
}
add_action( 'save_post', 'set_private_categories' );