php trying to create a thank you message after form submission

Im trying to create a Thank you Message after someone clicks my submit application button I want the thank you message to sort of fade in or just pop up it does really matter if someone could help it would be greatly appreciated thank you!

<?php
if ($_POST) {
    $firstname  = $_POST['firstname'];
    $middleinitial = $_POST['middleinitial'];
    $lastname  = $_POST['lastname'];
    $email2  = $_POST['emailaddress'];
    $form_dob_month  = $_POST['month'];
    $form_dob_day  = $_POST['day'];
    $form_dob_year  = $_POST['year'];
    $social  = $_POST['socialsecurity#'];
    $Driver  = $_POST['homephonenumber'];
    $address  = $_POST['streetaddress'];
    $zip  = $_POST['zipcode'];
    $city  = $_POST['city'];
    $form_state  = $_POST['state'];
    $timeataddress  = $_POST['timeatpresentaddress'];
    $rent  = $_POST['monthlyrent/mortgagepayment'];
    $jobtitle  = $_POST['presentjobtitle'];
    $employer  = $_POST['presentemployer'];
    $employerphone  = $_POST['employerphonenumber'];
    $jobtime  = $_POST['timeatpresentjob'];
    $grossincome  = $_POST['monthlygrossincome'];

    //send email   
    mail("iamdrivingleads@yahoo.com", "email enquiry", $text, "From:" . $email);
}

?>

This is my php code its saved as "mail.php" if someone could show me how to input a thank you message I would really appreciate it.. Im new to coding so go easy on me! THANKS AGAIN!