Untitled

<?php
// To send HTML mail, the Content-type header must be set
$headers = [];
$headers[] = 'MIME-Version: 1.0';
$headers[] = 'Content-type: text/html; charset=iso-8859-1';
$headers[] = 'From: Do Not Reply <noreply@example.com>';

$content = '
<html>
    <head>
        <style>
            p {
                color: red;
                font-size: 18px;
            }
        </style>
    </head>
    <body>
        <p>This is test message.</p>
    </body>
</html>
';

echo mail( 'johndoe@gmail.com', 'From Human', $content, implode("\r\n", $headers) ) . PHP_EOL;