You should use ob_get_contents()
ob_start();
include('template/email-header.php');
printf(__('<p>A very special welcome to you, %1$s. Thank you for joining %2$s!</p>', 'cell-email'), $greetings, get_bloginfo('name'));
printf(__('<p> Your password is <strong style="color:orange">%s</strong> <br> Please keep it secret and keep it safe! </p>', 'cell-email'), $plaintext_pass);
printf(__('<p>We hope you enjoy your stay at %s. If you have any problems, questions, opinions, praise, comments, suggestions, please feel free to contact us at any time</p>', 'cell-email'), get_bloginfo('name'));
include('template/email-footer.php');
$message = ob_get_contents();
ob_end_clean();
wp_mail($user_email, $email_subject, $message);
And on the template/email-header.php
, you can use
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta property="og:title" content="<?php echo $email_subject ?>" />
<title><?php echo $email_subject ?></title>
</head>
<body leftmargin="0" marginwidth="0" topmargin="0" marginheight="0" offset="0" style="width: 100% !important; -webkit-text-size-adjust: none; margin-top: 0; margin-right: 0; margin-bottom: 0; margin-left: 0; padding-top: 0; padding-right: 0; padding-bottom: 0; padding-left: 0; background-color: #FAFAFA;" bgcolor="#FAFAFA">
<!-- the rest of the html here -->
<?php // and php generated content if you prefer ?>