WordPress 4.4 introduced “title” as a “public query var”. This change doesn’t affect the average site, but does fill a gap by allowing you to query for a specific post by title. Previously you would have to know the post ID or slug, or use the WP search.
The reason we are writing about this on our blog is that many of you used our Register Helper Add On to add custom fields to your checkout, and have added “title” fields to checkout. Because these fields shared their name with the new title query var, if you are running WP 4.4 and have a custom field with the name “title” your checkout form will break.
We’ve fixed this in a couple of ways.
1. An update to the Register Helper Add On now adds a prefix to the name of the field at checkout, avoiding the conflict while still keeping the user meta field called “title”. Just updated your version of the plugin through your WordPress dashboard or grab the latest ZIP file here.
2. This gist can be added to your active theme’s functions.php or a custom plugin to disable “title” as a query var. This would be useful if you aren’t using Register Helper or can’t upgrade it or otherwise need to disable the new query var.
The WordPress core team is aware of the potential conflicts from this query var and future query vars and working on a solution, or at least a way to better test if new query vars are going to break functionality. You can follow the discussion on the WordPress trac here.