Use a select box to change a php variable

To store the user defined value into the DB you can use the update_user_meta() function. And retrieve back the info with get_user_meta(). You could update the the user meta key with an AJAX call on your select change or put your select in a form and update on form submit either with $_GET or $_POST method. Your choice here would influence if you need to build a function that listens on a $_POST, $_GET or AJAX request to update your user metadata.

You would also need to use wp_get_current_user to know the ID of the user to be supplied to the update_user_meta() and get_user_meta() functions.

Finally, using wp_login action hook, you could reinitialise to the default value of the current year by calling update_user_meta() again. Alternatively, you could do some actions on the wp_logout hook

Note that it is ok to use update_user_meta even if the key doesn’t exists, as the function will create it, if it doesn’t find a matching key for that user.

Check the wp_ajax_(action) hook to learn how to create your handler for your AJAX requests.

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