[Appointments +] - Send notifications for pending appointments

app-pending-notifications.php

<?php
/*
Plugin Name: [Appointments +] - Pending notifications
Plugin URI: https://premium.wpmudev.org/
Description: Send notifications for pending appointments
Author: Panos Lyrakis @ WPMUDEV
Author URI: https://premium.wpmudev.org/
License: GPLv2 or later
*/

add_action( 'wpmudev_appointments_insert_appointment', function( $app_id ){

    if( ! function_exists( 'appointments_get_appointment' ) ){
        return;
    }

    $app =              appointments_get_appointment( $app_id );

    if( 'pending' != $app->status ){
        return;
    }

    $appointments       = appointments();
    $service            = appointments_get_service( $app->service );
    $worker_name        = 'Our team';
    $customer_email     = $app->get_customer_email();    
    $content            = "<p>Hi {%USERNAME%}</p>";
    $content            .= "<p>You have booked an appointment for <strong>{%SERVICE_NAME%}</strong> on {%APP_DATE%} with {%WORKER_NAME%}. The appointment's price is $ {%APP_PRICE%} USD</p>";

    if( $app->worker ){
        $worker             = appointments_get_worker( $app->worker );
        $worker_name        = $worker->get_name( 'display_name' );    
    }

    $replacements       = array(
                            '{%USERNAME%}'      => $app->name,
                            '{%SERVICE_NAME%}'  => $service->name,
                            '{%APP_DATE%}'      => date( 'd, M Y H:i:s', strtotime( $app->start ) ),
                            '{%APP_PRICE%}'     => $app->price,
                            '{%WORKER_NAME%}'   => $worker_name
                        );

    $subject            = "You have a new appointment!";
    $email_body         = str_replace( 
                            array_keys( $replacements ), 
                            array_values( $replacements ), 
                            $content 
                        );

    $mail_to            = $customer_email = $app->get_customer_email();

    add_filter( 'app-emails-content_type', 'wpmudev_app_html_email_headers', 20 );

    wp_mail(
        $mail_to,
        $subject,
        $email_body,
        $appointments->message_headers()
    );

}, 20 );

function wpmudev_app_html_email_headers(){
    remove_filter( 'app-emails-content_type', 'app_html_email_headers', 20 );
    return 'text/html'; 
}