Here is the sample code for using output buffering. 

> *WordPress: Include template in the email.*

    ob_start();
    include(get_stylesheet_directory() . '/assets/email-templates/booking-details-template.php');
    $email_content = ob_get_contents();
    ob_end_clean();
    $headers = array('Content-Type: text/html; charset=UTF-8');
    wp_mail($to_email, "Booking details", $email_content, $headers);