public static function sanitize_settings( $settings = array() ) {

		$fields = self::fields();

		$fields = PUM_Admin_Helpers::flatten_fields_array( $fields );

		foreach ( $fields as $field_id => $field ) {
			switch ( $field['type'] ) {
				case 'checkbox':

					if ( ! isset( $settings[ $field_id ] ) ) {
						$settings[ $field_id ] = false;
					}
					break;
			}
		}

		foreach ( $settings as $key => $value ) {
			$field = self::get_field( $key );

			if ( $field ) {

				// Sanitize every string value.
				if ( is_string( $value ) ) {
					$settings[ $key ] = sanitize_text_field( $value );
				}

				switch ( $field['type'] ) {
					default:
						$settings[ $key ] = is_string( $value ) ? trim( $value ) : $value;
						break;


					case 'measure':
						$settings[ $key ] .= $settings[ $key . '_unit' ];
						break;

					case 'license_key':
						$old = PUM_Utils_Options::get( $key );
						$new = trim( $value );

						if ( $old && $old != $new ) {
							delete_option( str_replace( '_license_key', '_license_active', $key ) );
							call_user_func( $field['options']['activation_callback'] );
						}

						$settings[ $key ] = is_string( $value ) ? trim( $value ) : $value;
						// Activate / deactivate license keys maybe?
						break;
				}
			} else {
				// Some custom field types include multiple additional fields that do not need to be saved, strip out any non-whitelisted fields.
				unset( $settings[ $key ] );
			}
		}

		return $settings;
	}