通过php文件操作index.html中的HTML元素

Question

I made a simple contact form and I am trying to figure out how to change the html elements on the page to create a confirmation for when the e-mail is sent. As of now, it just reloads the page if an e-mail is sent. I would also like error messages to appear beside the input label as an html element if the name section is blank or e-mail entered is not valid.

This is the code that I have currently:

HTML:

<div id="contact-content" class="format-div">
    <form id="contact-form" action="/contact-form.php" method="post">
        <label for="contact-name">Name</label>
        <input type="text" id="contact-name" name="nameIn" placeholder="Enter 
       your name!">
        <label for="email-address">E-mail</label>
        <input type="text" id="email-address" name="emailIn" placeholder="Enter your e-mail address!">
        <label for="contact-message">Message</label>
        <textarea id="contact-message" name="messageIn" placeholder="All this space to fill with words..."
                  style="height:200px"></textarea>
        <button type="submit" name="submit-mail"><i class="far fa-paper-plane"></i></button>
    </form>
</div>

PHP:

if (isset($_POST['submit-mail'])) {

    $name = $_POST['nameIn'];
    $address = $_POST['emailIn'];
    $message = $_POST['messageIn'];
    $error = "";

    if($name == '') $error .= 'Please fill in your name';

    if (!filter_var($address, FILTER_VALIDATE_EMAIL)) $error .= 'The e-mail is invalid';

    $mailTo = "[e-mail]";
    $subject = "FROM WEBSITE: ".$name;
    $header = "Mail from: ".$name.", ".$address."

";

    mail($mailTo, $subject, $message, $header);
    header("Location: index.html?mailsent");
}

What I am going for with error messages

Answer 1

Inside a HTML tag you can put this: <php if (isset($_POST['submit-mail'])) { echo $error; } ?> so if theres an error it will show inside the HTML tag