add-plain-text-email.php

<?php
/*
Plugin Name: Add Plain Text Email
Plugin URI: https://www.dannyvankooten.com/
Description: Adds a text/plain email to text/html emails to decrease the chance of emails being tagged as spam.
Version: 1.2.1
Author: Danny van Kooten
Author URI: https://www.dannyvanKooten.com
Requires PHP: 7.4
License: GPL v3 or later
License URI: https://www.gnu.org/licenses/gpl-3.0.html

Add Plain Text Email
Copyright (C) 2013 Danny van Kooten

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/

defined( 'ABSPATH' ) OR exit;

class Add_Plain_Text_Email {

    /**
     * Add hooks
     */
    public function add_hooks() {
        add_action('phpmailer_init', array( $this, 'set_plaintext_body' ) );
    }

    /**
     * @param PHPMailer $phpmailer
     *
     * Note: somehow we can not type-hint the parameter here as PHPMailer...
     */
    public function set_plaintext_body( $phpmailer ) {

        // don't run if sending plain text email already
        if( $phpmailer->ContentType === 'text/plain' ) {
            return;
        }

        // don't run if AltBody is set (by other plugin)
        if( ! empty( $phpmailer->AltBody ) ) {
            return;
        }

        // set AltBody
        $phpmailer->AltBody = $this->strip_html_tags( $phpmailer->Body );
    }

    /**
     * Remove HTML tags, including invisible text such as style and
     * script code, and embedded objects.  Add line breaks around
     * block-level tags to prevent word joining after tag removal.
     */
    public function strip_html_tags( $text ) {
        $text = preg_replace(
            array(
              // Remove invisible content
                '@<head[^>]*?>.*?</head>@siu',
                '@<style[^>]*?>.*?</style>@siu',
                '@<script[^>]*?.*?</script>@siu',
                '@<object[^>]*?.*?</object>@siu',
                '@<embed[^>]*?.*?</embed>@siu',
                '@<noscript[^>]*?.*?</noscript>@siu',
                '@<noembed[^>]*?.*?</noembed>@siu',
                '@\t+@su',
                '@\n+@su'
            ),
            '',
            $text );

        // replace certain elements with a line-break
        $text = preg_replace(
            array(
                '@</?((div)|(h[1-9])|(/tr)|(p)|(pre))@iu'
            ),
            "\n\$0",
            $text );

        // replace other elements with a space
        $text = preg_replace(
            array(
                '@</((td)|(th))@iu'
            ),
            " \$0",
            $text );

        // preserve the href attribute from <a> elements
        $text = preg_replace( '@<a[^>]+href=["\']([^"\']+)[^>]+>(.+)</a>@iu', "\$2 (\$1)", $text );

        // strip all remaining HTML tags
        $text = strip_tags( $text );

        return  trim( $text );
    }

}

$apte = new Add_Plain_Text_Email;
$apte->add_hooks();