Require discount code on checkout

require_discount_code.php

<?php

// Copy from below here

/*
 * Require that a user use a discount code when reigstering for specific levels.
 */
function my_pmpro_registration_checks_require_code_to_register( $pmpro_continue_registration ) {
	$restricted_levels = array( 1, 3 ); // Levels 1 and 3 require a discount code to register
	global $discount_code, $pmpro_level;

	if( empty( $discount_code ) && in_array( $pmpro_level->id, $restricted_levels ) ) {
		pmpro_setMessage("You must use a valid discount code to register for this level.", "pmpro_error");
		return false;
	}

	return $pmpro_continue_registration;
}
add_filter("pmpro_registration_checks", "my_pmpro_registration_checks_require_code_to_register");