Sending an html email is not much different from sending normal emails using php. What is necessary to add is the content type along the header parameter of the php mail() function. Here is an example.
<?php
$to = "[email protected]";
$subject = "HTML email";
$message = "
<html>
<head>
<title>HTML email</title>
</head>
<body>
<p>A table as email</p>
<table>
<tr>
<th>Firstname</th>
<th>Lastname</th>
</tr>
<tr>
<td>Fname</td>
<td>Sname</td>
</tr>
</table>
</body>
</html>
";
// Always set content-type when sending HTML email
$headers = "MIME-Version: 1.0" . "\r\n";
$headers .= "Content-type:text/html;charset=UTF-8" . "\r\b";
$headers .= 'From: name' . "\r\n";
mail($to,$subject,$message,$headers);
?>
You can also check here for more detailed explanations by w3schools