WordPress at a glance
function is not described

PHPMailer\PHPMailer

PHPMailer::isHTML() public WP 1.0

Sets message type to HTML or plain.

{} It's a method of the class: PHPMailer{}

No Hooks.

Return

null. Nothing.

Usage

$PHPMailer = new PHPMailer();
$PHPMailer->isHTML( $isHtml );
$isHtml(true|false)
True for HTML mode

Code of PHPMailer::isHTML() WP 5.7.2

<?php
public function isHTML($isHtml = true)
{
    if ($isHtml) {
        $this->ContentType = static::CONTENT_TYPE_TEXT_HTML;
    } else {
        $this->ContentType = static::CONTENT_TYPE_PLAINTEXT;
    }
}