Ultimate Member + Country State City Dropdown PRO: Custom callbacks for select fields Country, State, City

functions.php

function um_customcb_populate_countries() {
	$countries = array();
	if ( ! function_exists( 'get_countries' ) ) {
		return $countries;
	}

	$all_countries = get_countries();
	if ( empty( $all_countries ) ) {
		return $countries;
	}

	foreach ( $all_countries as $country ) {
		$countries[ $country->id ] = $country->name;
	}
	return $countries;
}

function um_customcb_populate_states() {
	$states = array();
	if ( empty( $_POST['parent_option'] ) ) {
		return $states;
	}

	if ( ! function_exists( 'get_states_by_country_id' ) ) {
		return $states;
	}

	if ( is_array( $_POST['parent_option'] ) ) {
		$choices = array_map( 'absint', $_POST['parent_option'] );
	} else {
		$choices = array( absint( $_POST['parent_option'] ) );
	}

	foreach ( $choices as $choice ) {
		$all_states = get_states_by_country_id( $choice );
		if ( empty( $all_states ) ) {
			continue;
		}

		foreach ( $all_states as $state ) {
			$states[ $state->id ] = $state->name;
		}
	}

	return $states;
}

function um_customcb_populate_city() {
	$cities = array();
	if ( empty( $_POST['parent_option'] ) ) {
		return $cities;
	}

	if ( ! function_exists( 'get_cities_by_state_id' ) ) {
		return $cities;
	}

	if ( is_array( $_POST['parent_option'] ) ) {
		$choices = array_map( 'absint', $_POST['parent_option'] );
	} else {
		$choices = array( absint( $_POST['parent_option'] ) );
	}

	foreach ( $choices as $choice ) {
		$all_cities = get_cities_by_state_id( $choice );
		if ( empty( $all_cities ) ) {
			continue;
		}
		foreach ( $all_cities as $city ) {
			$cities[ $city->id ] = $city->name;
		}
	}

	return $cities;
}