Send Notification When a Submitted Event is Published

<?php
/*
Plugin Name: Tribe Snippet: Send email notification when the event is approved
Requires PHP: 5.6
Version: 0.1
 */

add_action( 'transition_post_status', function ( $new_status, $old_status, $post ) {
	// Early bail: We are looking for published posts only
	if ( $new_status != 'publish' ) {
		return;
	}

	// Early bail: Unexpected value
	if ( ! $post instanceof WP_Post ) {
		return;
	}

	// Early bail: We need the "tribe" function to be loaded
	if ( ! function_exists( 'tribe' ) ) {
		return;
	}

	$main = tribe( 'community.main' );

	// Early bail: We could not get the desired class from the Container
	if ( ! $main instanceof Tribe__Events__Community__Main ) {
		return;
	}

	$default_status = $main->getOption( 'defaultStatus' );

	// Early bail: Old status is not the default status
	if ( $old_status != $default_status ) {
		return;
	}

	// Early bail: We are just interested in Events
	if ( $post->post_type != 'tribe_events' ) {
		return;
	}

	$author = get_user_by( 'id', $post->post_author );

	// Early bail: Author not available (eg: Anonymous submission)
	if ( ! $author instanceof WP_User ) {
		return;
	}

	$email = $author->user_email;

	// Early bail: Invalid email
	if ( ! filter_var( $email, FILTER_VALIDATE_EMAIL ) ) {
		return;
	}

	$subject = sprintf( 'Congratulations! %s was published!', $post->post_title );
	$message = sprintf( 'Congratulations, your event was published! Click <a href="%s">here</a> to view it!', esc_url( get_post_permalink( $post ) ) );

	wp_mail(
		$email,
		esc_html__( $subject, 'tribe-events-community' ),
		__( wp_kses_post_deep( $message ), 'tribe-events-community' ),
		[ 'Content-Type: text/html; charset=UTF-8' ]
	);

}, 10, 3 );