Simple Wordpress mail settings plugin

mailer.php

<?php
/*
 * Plugin Name:       SMTP Mail Setup
 */

// Error when called directly
if (!defined('ABSPATH')) {
    die();
}

// Return env variable or return default value
function get_env_value($env, $default) {
    $value = getenv($env);
    if ($value !== false) {
        return $value;
    }
    return $default;
}

// Return value from env_FILE or fallback to env value or default
function get_env_value_file($env, $default) {
    $filename = getenv($env . '_FILE');
    if ($filename !== false) {
        $content = file_get_contents($filename);
        if ($content !== false) {
            return rtrim($content, "\r\n");
        }
    }
    return get_env_value($env, $default);
}

function mailer_filter_custom_wp_mail_from( $address ) {
    $smtp_from = get_env_value("MAILER_SMTP_FROM", '');

    if (!empty($smtp_from))
        return $smtp_from;

    return $address;
}

function mailer_custom_phpmailer_init( $phpmailer ) {
    $smtp_host = get_env_value("MAILER_SMTP_HOST", '');
    if (empty($smtp_host)) {
        return;
    }

    $smtp_port = get_env_value("MAILER_SMTP_PORT", 465);
    $smtp_user = get_env_value_file("MAILER_SMTP_USER", '');
    $smtp_pass = get_env_value_file("MAILER_SMTP_PASS", '');
    $smtp_starttls = get_env_value("MAILER_SMTP_STARTTLS", false);

    // Send using SMTP
    $phpmailer->isSMTP();
    $phpmailer->Host = $smtp_host;
    $phpmailer->Port = $smtp_port;

    if (!empty($smtp_user) || !empty($smtp_pass)) {
        $phpmailer->SMTPAuth = true;
        if (!empty($smtp_user)) {
            $phpmailer->Username = $smtp_user;
        }
        if (!empty($smtp_pass)) {
            $phpmailer->Password = $smtp_pass;
        }
    }
    $phpmailer->SMTPSecure = PHPMailer\PHPMailer\PHPMailer::ENCRYPTION_SMTPS;

    if (filter_var($smtp_starttls, FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE)) {
        $phpmailer->SMTPSecure = PHPMailer\PHPMailer\PHPMailer::ENCRYPTION_STARTTLS;
    }
}

add_action( 'phpmailer_init',   'mailer_custom_phpmailer_init' );
add_filter( 'wp_mail_from',     'mailer_filter_custom_wp_mail_from' );