WP Simple Pay: Custom Server-Side Validation Before Payment Creation

/**

 * @link https://library.wpcode.com/snippet/ro8wgkow/

 */

add_action(

	'simpay_before_payment_create',

	/**

	 * @param \WP_REST_Request WordPress REST API request.

	 */

	function( $request ) {

		// "Stripe Metadata Label" of the custom field.

		$custom_field_name = 'Customer DOB'; 

		// Retrieve form values.

		$fields = $request->get_param( 'form_values' );

		// Field not found.

		if ( ! isset( $fields['simpay_field'][ $custom_field_name ] ) ) {

			throw new \Exception( 'You must be 18 years or older to purchase.' );

		}

		// Custom validation.

		$dob = strtotime( intval( $fields['simpay_field'][ $custom_field_name ] ) );

		if ( false === $dob || $dob > strtotime( '-18 years' ) ) {

			throw new \Exception( 'You must be 18 years or older to purchase.' );

		}

		// Payment can be created.

	}

);