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?