Wordpress Change Default Display Name Publicy As for all existing users

Question

does anyone know of way I can change the existing display name publicly as for all users. I want to have firstname lastname as the default because this will reflect in the forum. I have read all the forums and have tried all the hacks, any suggestions would be appreciated. Thanks in advance

Answer 1

Here's an improved version of richplane's answer that works in newer versions of WordPress (3.8+):

/**
* Format WordPress User's "Display Name" to Full Name on Login
* ------------------------------------------------------------------------------
*/

add_action( 'wp_login', 'wpse_9326315_format_user_display_name_on_login' );

function wpse_9326315_format_user_display_name_on_login( $username ) {
    $user = get_user_by( 'login', $username );

    $first_name = get_user_meta( $user->ID, 'first_name', true );
    $last_name = get_user_meta( $user->ID, 'last_name', true );

    $full_name = trim( $first_name . ' ' . $last_name );

    if ( ! empty( $full_name ) && ( $user->data->display_name != $full_name ) ) {
        $userdata = array(
            'ID' => $user->ID,
            'display_name' => $full_name,
        );

        wp_update_user( $userdata );
    }
}

Answer 2

Problem with using the admin_head hook is that it doesn't work for users who don't use the admin system. Also, my attempts to implement the solution posted by Marty failed because it doesn't seem that the display_name can be updated by update_user_meta() - you have to use wp_update_user().

My proposal - put this in your functions.php file:

function force_pretty_displaynames($user_login, $user) {

    $outcome = trim(get_user_meta($user->ID, 'first_name', true) . " " . get_user_meta($user->ID, 'last_name', true));
    if (!empty($outcome) && ($user->data->display_name!=$outcome)) {
        wp_update_user( array ('ID' => $user->ID, 'display_name' => $outcome));    
    }
}
add_action('wp_login','force_pretty_displaynames',10,2); 

For me (using WP 3.4.1), that works OK, replacing the display name as soon as they log in.