What is the best practice for including a .tpl file in PHP and how can template variables be populated with values?

When including a .tpl file in PHP, it is best practice to use the `include` or `require` function to include the template file. To populate template variables with values, you can use PHP's `echo` or `print` functions within the template file to output the variable values.

<?php
// Include the template file
require 'template.tpl';

// Define template variables
$name = 'John Doe';
$email = 'john.doe@example.com';

// Output the template variables within the template file
echo "Name: $name";
echo "Email: $email";
?>