Prevent author from changing their posts if admin has modified

Question

In wordpress, by default, authors can create and make all changes to their posts.

I'm looking for a way to restrict them from making to any change to a post if administrator has made any modification to that particular post (after the admin click "update" button).

For posts that have not been modified by administrator, the author still can make any change they want.

I have tried through a number of user role plugins, but cannot find any plugin that can help.

Thank you very much.

Answer 1

You can try the following:

/**
 * Post Update Locker For Authors
 * If an administrator has updated the post, then lock it for author updates.
 * @see http://wordpress.stackexchange.com/a/168578/26350
 */

add_action( 'pre_post_update', function( $post_ID, $data ) {

    // Target only authors:
    if( ! current_user_can( 'edit_post' ) || current_user_can( 'edit_others_posts' ) )      
        return;

    // Target only 'post' post types:
    if( get_post_type( $post_ID ) !== 'post' )
        return;

    // Fetch all administrators:
    $admins_ids = get_users( 
        array( 
            'role'   => 'administrator', 
            'fields' => 'ID' 
        ) 
    );

    // or hardcoded if needed:
    // $admins_ids = array( 1 );

    // Check if administrators have modified the current post, by checking the revisions:
    $posts = get_posts( 
        array( 
            'no_found_rows'             => true,
            'update_post_meta_cache'    => false,
            'update_post_term_cache'    => false,
            'posts_per_page'            => -1, 
            'fields'                    => 'ids', 
            'post_parent'               => $post_ID, 
            'post_type'                 => 'revision', 
            'post_status'               => 'any', 
            'author__in '               => $admin_ids,  
        ) 
    );

    // Halt if an administrator has modified the post:
    if( count( $posts ) > 0 )
        wp_die( __( "Sorry, you can't modify this post, it's already been modified by an administrator! " ) );

}, 10, 2 );

You might have to adjust this and test further.

Update:

Here's an additional example how one can remove the submitdiv metabox for authors that can't update the current post anymore:

/**
 * Hide the publish metabox for authors if an administrator has updated the post.
 * @see http://wordpress.stackexchange.com/a/168578/26350
 */

add_action( 'admin_menu', function() {
    add_action( 'load-post.php', 'wpse_author_update_locking' );
});

function wpse_author_update_locking()
{
    // Setup - Modify this to your needs:
    $admin_ids = array( 1 );
    $cpt       = 'post';

    // User input:
    $_pid    = filter_input( INPUT_GET, 'post',   FILTER_SANITIZE_NUMBER_INT );

    // Target only $cpt post types:
    if( get_post_type( $_pid ) !== $cpt )
        return;

    // Target only authors:
    if( ! current_user_can( 'edit_post' ) || current_user_can( 'edit_others_posts' ) )      
        return;

    if( $_pid > 0 )
    {
        // Check if administrators have modified the current post, 
        // by checking the revisions:
        $posts = get_posts( 
            array( 
                'no_found_rows'             => true,
                'update_post_meta_cache'    => false,
                'update_post_term_cache'    => false,
                'posts_per_page'            => -1, 
                'fields'                    => 'ids', 
                'post_parent'               => $_pid, 
                'post_type'                 => 'revision', 
                'post_status'               => 'any', 
                'author__in '               => $admin_ids,  
            ) 
        );

        // Halt if an administrator has modified the post:
        if( count( $posts ) > 0 )
        {
            remove_meta_box( 'submitdiv', $cpt , 'side' ); 
            add_meta_box( 
                'submitdivmod', 
                __( 'Publish' ), 
                'wpse_post_submit_meta_box', 
                $cpt, 
                'side', 
                'core' 
            ); 
        }
    }
}

function wpse_post_submit_meta_box()
{
    _e("Sorry, you can't modify this post, it's already been modified by an administrator!");
}

and then instead of this metabox:

the author sees this: