I have created a basic form which can be POST to admin-post.php and also submitted via an AJAX call to admin-ajax.php. Both submissions work correctly, use the nonce for a little extra security, and I can process the data as required.

My question and problem is how, with the regular form POST to admin-post.php, to send back a message to the form with either a success message string, or an error – which could be a string, or an array of invalid form field entries? With the AJAX call I can simply echo a json encoded response.

I know there is no session set unless I manually call session_start() and also using globals isn’t ideal. Is there some other sort of storage or system I can use to pass these messages?

I’ve used transients before – would these be suitable? I understand the transient data may not be available at any given moment so perhaps not?

Could I use the WP_Error class?