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1<?php
2/**
3 * Set up custom priority headers for outbound email sent via SMTP.
4 * See pMz3w-gAF-p2#comment-104972 for the specific header structure and
5 * background discussion for our initial use cases.
6 *
7 * @since 3.5.36
8 * @package wpcomsh
9 */
10
11/**
12 * Maximum priority level.
13 */
14const WPCOMSH_SMTP_LEVEL_MAX = 9;
15/**
16 * Minimum priority level.
17 */
18const WPCOMSH_SMTP_LEVEL_MIN = 1;
19/**
20 * The default level for users with reduced email priority.
21 */
22const WPCOMSH_SMTP_LEVEL_REDUCED = 2;
23/**
24 * A slightly increased level for emails that should get priority, like password resets and new user registrations.
25 */
26const WPCOMSH_SMTP_LEVEL_LOCAL_PRIORITY = 5;
27
28/**
29 * Add the custom email priority SMTP header if that's something we need for this site.
30 *
31 * @param \PHPMailer\PHPMailer\PHPMailer $phpmailer The current PHPMailer instance, which is passed by reference.
32 * @return void
33 * @throws \PHPMailer\PHPMailer\Exception Exception may be thrown by the {@see PHP_Mailer::addCustomHeader()} function.
34 */
35function wpcomsh_smtp_add_priority_header( $phpmailer ): void {
36    if ( ! wpcom_site_has_feature( \WPCOM_Features::REDUCED_ATOMIC_EMAIL_PRIORITY ) ) {
37        return;
38    }
39
40    /**
41     * Filters the SMTP email priority.
42     *
43     * @since 3.5.36
44     *
45     * @param int|null  $priority The priority we should apply. Default is 2.
46     * @return int|null Return null if you want no priority. Otherwise the returned value must be 0-9.
47     */
48    $atomic_email_priority = apply_filters( 'wpcomsh_smtp_email_priority', WPCOMSH_SMTP_LEVEL_REDUCED );
49
50    // If the priority is null, assume we don't want any priority.
51    if ( null === $atomic_email_priority ) {
52        return;
53    }
54
55    // Ensure we have an integer between 1-9; if we don't have something int-like, use the default priority.
56    if ( ! is_int( $atomic_email_priority ) ) {
57        $atomic_email_priority = WPCOMSH_SMTP_LEVEL_REDUCED;
58    }
59    $atomic_email_priority = min( WPCOMSH_SMTP_LEVEL_MAX, $atomic_email_priority );
60    $atomic_email_priority = max( WPCOMSH_SMTP_LEVEL_MIN, $atomic_email_priority );
61
62    $phpmailer->addCustomHeader( 'X-Atomic-Email-Level', $atomic_email_priority );
63}
64
65// Run this relatively late to make sure standard SMTP configuration has already been completed
66add_action( 'phpmailer_init', 'wpcomsh_smtp_add_priority_header', 100 );
67
68/**
69 * Helper function to return the local priority SMTP level - {@see WPCOMSH_SMTP_LEVEL_LOCAL_PRIORITY}.
70 *
71 * @since 3.5.36
72 */
73function wpcomsh_smtp_set_priority_local_priority(): int {
74    return WPCOMSH_SMTP_LEVEL_LOCAL_PRIORITY;
75}
76
77/**
78 * Helper function to ensure that we add a filter that sets the SMTP email priority to {@see WPCOMSH_SMTP_LEVEL_LOCAL_PRIORITY};
79 *
80 * @since 3.5.36
81 */
82function wpcomsh_smtp_add_local_priority_filter(): void {
83    add_filter( 'wpcomsh_smtp_email_priority', 'wpcomsh_smtp_set_priority_local_priority' );
84}
85
86/**
87 * Helper function to allow us to set up local priority from within a filter.
88 * {@see wpcomsh_smtp_add_local_priority_filter()}.
89 *
90 * @since 3.5.36
91 *
92 * @param mixed $filtered_value The value for the filter. We return this as-is from the function.
93 * @return mixed
94 */
95function wpcomsh_smtp_add_local_priority_filter_within_filter( $filtered_value ) {
96    wpcomsh_smtp_add_local_priority_filter();
97
98    return $filtered_value;
99}
100
101// Ensure emails sent as a result of password resets get local priority.
102add_action( 'password_reset', 'wpcomsh_smtp_add_local_priority_filter' );
103// Ensure emails sent as a result of a new user registration get local priority.
104add_action( 'register_new_user', 'wpcomsh_smtp_add_local_priority_filter' );
105// Ensure emails sent as part of a password retrieval/reset get local priority.
106add_filter( 'retrieve_password_notification_email', 'wpcomsh_smtp_add_local_priority_filter_within_filter' );