Validating a new user registration field

What I’m trying to do:

• Add new user meta fields that can be updated when the user edits their profile
• Validate them correctly when the form is submitted

What I’ve done:

I have successfully added the fields to the user editting form and have them being updated to the options database as follows:

public function setup_user_meta() {
    // Show the fields when editing or showing
    add_action('show_user_profile', array($this,'show_user_meta'));
    add_action('edit_user_profile', array($this,'show_user_meta'));
    // Update the fields
    add_action('personal_options_update', array($this,'process_option_update'));
    add_action('edit_user_profile_update', array($this,'process_option_update'));
}

.

public function show_user_meta($user) {
    ?>

<h3>Player Information</h3>
<table class="form-table">
    <tbody>
        <tr>
            <th><label for="cuhc-description">Description</label></th>
            <td><textarea name="cuhc-description" id="cuhc-description" rows="5" cols="30"><?php echo esc_textarea(get_the_author_meta('cuhc-description', $user->ID)); ?></textarea>
        </tr>
    </tbody>
</table>

<?php
    }

.

    public function process_option_update($userid) {
        update_user_meta($userid, 'cuhc-description', ( isset($_POST['cuhc-description']) ? $_POST['cuhc-description'] : '' ) );
    }

What I want:

This works, however I have a number of different fields and I want to validate them in process_option_update(), eg: checking they’re a number and if not returning an error to the user.

How do I tell WordPress there is an error and not to continue to act on the form? And where do I echo or send the error(s)? WordPress also appears to use javascript validation within the user update page – how do I piggy-back onto this instead of doing my own one?

After examining the code and much trial and error, I have a solution:

There is a filter – user_profile_update_errors – that is called in the file wp-admin/includes/user.php just after WordPress does it’s own input validation. This filter allows plugins to return their own errors by adding them to a referenced WP_Error class. If any errors exist after the filter returns, WordPress displays them and does not insert/update any data.

My solution is to therefore ditch the personal_options_update and edit_user_profile_update action hooks (as these are called BEFORE the user_profile_update_errors filter, immediately after the form is submitted) and to use the user_profile_update_errors filter to:

• retrieve any inputted data from $_POST,
• validate it,
• return any errors,
• and if non, update the options database with update_user_meta($ID,
$key, $value);.

Alternatively, one could use the profile_update or user_register action hooks (these are called near the end, just after the WordPress user data has been added to the database) to add our custom data to the options database. This splits up the validation and the database insertion, but data would have to be passed between the validation and insertion functions with a global stdClass or array, which I considered more ugly.

I hope this helps someone else!