Add the reason for cancelling to the email body only when the shortcode !!reason!! was not used. #pmpro-reason-for-cancelling

pmpror4c-conditional-email-body-string-replace.php

<?php
/**
 * This recipe adds the reason for cancelling in the default cancel email templates
 * if the email shortcode !!reason!! is not in the template.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpror4c_pmpro_email_body( $body, $email ) {
	if ( ! empty( $_REQUEST['reason'] ) ) {
		$reason = wp_unslash( sanitize_text_field( $_REQUEST['reason'] ) );
	} else {
		$reason = __( 'N/A', 'pmpro-reason-for-cancelling' );
	}

	// replace in standard templates if !!reason!! shortcode not present in template
	if ( $email->template == 'cancel' || $email->template == 'cancel_admin' ) {
		if ( ! strpos( $body, '!!reason!!' ) ) {
			$body = str_replace( 'has been cancelled.</p>', 'has been cancelled.</p><p>Reason: ' . $reason . '</p>', $body );
		}
	}

	// or replace in custom template
	$body = str_replace( '!!reason!!', $reason, $body );

	return $body;
}

function my_init_pmpror4c_pmpro_email_body_action() {
	remove_action( 'pmpro_email_body', 'pmpror4c_pmpro_email_body', 10, 2 );
	add_action( 'pmpro_email_body', 'my_pmpror4c_pmpro_email_body', 10, 2 );
}
add_action( 'init', 'my_init_pmpror4c_pmpro_email_body_action' );