gistfile.php

<?php
/** @link https://samelh.com/blog/2016/12/22/add-custom-fields-bbpress-profile/ **/

// add field to bbP profile edit
add_action( "bbp_user_edit_after_contact", "se_add_city_field" );

function se_add_city_field() {
    // a random selection of cities
    $cities = array( "Seoul", "Mexico City", "Amsterdam", "Mumbai", "Agadir", "Egypt" );
    // selected city, if any
    $user_city = bbp_get_displayed_user_field('se_city');
    ?>
    <div>
        <label for="city"><?php _e( 'City', 'my-domain' ); ?></label>
        <select name="city" id="city">
            <option value="">Select City</option>
            <?php foreach ( $cities as $city ) : ?>
                <option <?php selected($user_city,$city); ?>><?php echo esc_attr( $city ); ?></option>
            <?php endforeach; ?>
        </select>
    </div>
    <?php
}

// when updating our profile
add_action( "personal_options_update", "se_save_city_value" );
// when updating other users' profiles
add_action( "edit_user_profile_update", "se_save_city_value" );

function se_save_city_value( $user_id ) {
    // exclude profile.php/user-edit update
    if ( is_admin() ) return;
    // update preference
    if ( isset( $_POST['city'] ) && $_POST['city'] ) {
        return update_user_meta( $user_id, "se_city", sanitize_text_field( $_POST['city'] ) );
    } else {
        return delete_user_meta( $user_id, "se_city" );
    }
}

// add city after user profile details
add_action( "bbp_template_after_user_profile", "se_embed_city_profile" );

function se_embed_city_profile() {
    // selected city, if any
    $user_city = bbp_get_displayed_user_field('se_city');
    // making sure user has set their city first
    if ( !trim( $user_city ) ) return;
    // all good
    return printf(
        '<p>%s is based on <strong>%s</strong></p>',
        esc_attr( bbp_get_displayed_user_field('nickname') ),
        esc_attr( $user_city )
    );
}

// add city after topic/reply author details
add_action( "bbp_theme_after_reply_author_details", "se_embed_city_forums" );

function se_embed_city_forums() {
    // topic/reply author
    $user_id = bbp_get_reply_author_id();
    // selected city, if any
    $user_city = get_user_meta( $user_id, 'se_city', true );
    // making sure user has set their city first
    if ( !trim( $user_city ) ) return;
    // all good
    return printf(
        '<p>%s is based on <strong>%s</strong></p>',
        esc_attr( bbp_get_displayed_user_field('nickname') ),
        esc_attr( $user_city )
    );
}