Source: includes/emails/class-sensei-email-teacher-new-message.php

<?php
// phpcs:disable VariableAnalysis.CodeAnalysis.VariableAnalysis -- Prevent "Unused global variable $sensei_email_data"
if ( ! defined( 'ABSPATH' ) ) {
	exit; // Exit if accessed directly
}

if ( ! class_exists( 'Sensei_Email_Teacher_New_Message', false ) ) :

	/**
	 * Teacher New Message
	 *
	 * An email sent to the teacher when one of their students sends them a private message.
	 *
	 * @package Users
	 * @author Automattic
	 *
	 * @since       1.6.0
	 */
	class Sensei_Email_Teacher_New_Message {

		var $template;
		var $subject;
		var $heading;
		var $recipient;
		var $learner;
		var $teacher;
		var $message;

		/**
		 * Constructor
		 */
		function __construct() {
			$this->template = 'teacher-new-message';
		}

		/**
		 * trigger function.
		 *
		 * @access public
		 * @param integer $message_id
		 * @return void
		 */
		function trigger( $message_id = 0 ) {
			global  $sensei_email_data;

			$this->message = get_post( $message_id );

			$learner_username = get_post_meta( $message_id, '_sender', true );
			$this->learner    = get_user_by( 'login', $learner_username );

			$teacher_username = get_post_meta( $message_id, '_receiver', true );
			$this->teacher    = get_user_by( 'login', $teacher_username );

			// Set recipient (teacher)
			$this->recipient = stripslashes( $this->teacher->user_email );

			/**
			 * Action hook before email is sent.
			 *
			 * @hook sensei_before_mail
			 *
			 * @param {string} $recipient The recipient email.
			 */
			do_action( 'sensei_before_mail', $this->recipient );

			// translators: Placeholder is the blog name.
			$this->subject = apply_filters( 'sensei_email_subject', sprintf( __( '[%1$s] You have received a new private message', 'sensei-lms' ), get_bloginfo( 'name' ) ), $this->template );
			$this->heading = apply_filters( 'sensei_email_heading', __( 'Your student has sent you a private message', 'sensei-lms' ), $this->template );

			$content_type  = get_post_meta( $message_id, '_posttype', true );
			$content_id    = get_post_meta( $message_id, '_post', true );
			$content_title = get_the_title( $content_id );

			// setup the post type parameter
			$content_type = get_post_type( $content_id );
			if ( ! $content_type ) {
				$content_type = '';
			}

			// Construct data array
			$sensei_email_data = apply_filters(
				'sensei_email_data',
				array(
					'template'            => $this->template,
					$content_type . '_id' => $content_id,
					'heading'             => $this->heading,
					'teacher_id'          => $this->teacher->ID,
					'learner_id'          => $this->learner->ID,
					'learner_name'        => $this->learner->display_name,
					'message_id'          => $message_id,
					'message'             => $this->message->post_content,
					'content_title'       => $content_title,
					'content_type'        => $content_type,
				),
				$this->template
			);

			// Send mail
			Sensei()->emails->send( $this->recipient, $this->subject, Sensei()->emails->get_content( $this->template ) );

			/**
			 * Action hook after email is sent.
			 *
			 * @hook sensei_after_sending_email
			 */
			do_action( 'sensei_after_sending_email' );
		}
	}

endif;