Compare with other filed validation

/**
 * Can validate field by comparing value with other field.
 */
class WPDesk_FCF_Validation_Confirm_Field {
	/**
	 * Field to compare with validated field.
	 *
	 * @var string
	 */
	private $field_to_compare;
	/**
	 * WPDesk_FCF_Validation_Confirm_Field constructor.
	 *
	 * @param string $field_to_compare .
	 */
	public function __construct( $field_to_compare ) {
		$this->field_to_compare = $field_to_compare;
	}
	public function hooks() {
		add_filter( 'flexible_checkout_fields_custom_validation', array( $this, 'register_custom_validation' ) );
	}
	/**
	 * Register custom validation.
	 *
	 * @param array $custom_validation .
	 *
	 * @return array
	 */
	public function register_custom_validation( $custom_validation ) {
		$custom_validation[ 'field_confirmation_' . $this->field_to_compare ] = array(
			'label'    => sprintf( __( 'Compare with %1$s', 'wpdesk' ), $this->field_to_compare ),
			'callback' => array( $this, 'validate' )
		);
		return $custom_validation;
	}
	/**
	 * Validate.
	 *
	 * @param string $field_label Field label.
	 * @param string $value Field value.
	 */
	public function validate( $field_label, $value ) {
		$field_to_compare_value = sanitize_text_field( $_POST[ $this->field_to_compare ] );
		$valid = $field_to_compare_value === $value;
		if ( ! $valid ) {
			wc_add_notice( sprintf( __( 'Invalid %1$s value.', 'wpdesk' ), '<strong>' . $field_label . '</strong>' ), 'error' );
		}
	}
}
$fcf_validation_confirm_field = new WPDesk_FCF_Validation_Confirm_Field( 'billing_email' );
$fcf_validation_confirm_field->hooks();