How do I make contact form disappear onsubmit?

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PHP

forms

I am trying to make a form that will hide on submit and replace it with a thank you msg. Ive tried several things but nothing has worked so far. The code I have now works fine. I would ultimately like the error msg to show on the same page as well with a back button. I am working on that for now. Wish me luck! Thanks for all the help in advance you guys are always awesome!

this is my php

if (isset($_POST['email'])) {
    $email_to = "stormygurl73@yahoo.com";
    $email_subject = "Your email subject line";

    function died($error) {
        echo "We are very sorry, but there were error(s) found with the form you submitted. ";
        echo "These errors appear below.<br /><br />";
        echo $error."<br /><br />";
        echo "Please go back and fix these errors.<br /><br />";
        die();
    }
    // validation expected data exists
    if (!isset($_POST['first_name']) || !isset($_POST['last_name']) || !isset($_POST['email']) || !isset($_POST['telephone']) || !isset($_POST['comments'])) {
        died('We are sorry, but there appears to be a problem with the form you submitted.');
    }
    $first_name = $_POST['first_name']; // required
    $last_name = $_POST['last_name']; // required
    $email_from = $_POST['email']; // required
    $telephone = $_POST['telephone']; // not required
    $comments = $_POST['comments']; // required
    $error_message = "";
    $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
    if (!preg_match($email_exp, $email_from)) {
        $error_message. = 'The Email Address you entered does not appear to be valid.<br />';
    }
    $string_exp = "/^[A-Za-z .'-]+$/";
    if (!preg_match($string_exp, $first_name)) {
        $error_message. = 'The First Name you entered does not appear to be valid.<br />';
    }
    if (!preg_match($string_exp, $last_name)) {
        $error_message. = 'The Last Name you entered does not appear to be valid.<br />';
    }
    if (strlen($comments) < 2) {
        $error_message. = 'The Comments you entered do not appear to be valid.<br />';
    }
    if (strlen($error_message) > 0) {
        died($error_message);
    }
    $email_message = "Form details below.\n\n";

    function clean_string($string) {
        $bad = array("content-type", "bcc:", "to:", "cc:", "href");
        return str_replace($bad, "", $string);
    }
    $email_message. = "First Name: ".clean_string($first_name)."\n";
    $email_message. = "Last Name: ".clean_string($last_name)."\n";
    $email_message. = "Email: ".clean_string($email_from)."\n";
    $email_message. = "Telephone: ".clean_string($telephone)."\n";
    $email_message. = "Comments: ".clean_string($comments)."\n";
    $headers = 'From: '.$email_from."\r\n".'Reply-To: '.$email_from."\r\n".'X-Mailer: PHP/'.phpversion();@mail($email_to, $email_subject, $email_message, $headers);
    echo "We have received your enquiry. We will contact you soon.";
} ?>

I am using just a basic contact form. Thanks for any help!