How can PHP be used to highlight unfilled fields in a form for user attention?

When a user submits a form with unfilled fields, it can be helpful to highlight these fields to draw their attention and prompt them to fill in the required information. This can be achieved using PHP by checking if the form has been submitted, then iterating through the form fields and adding a CSS class to highlight any unfilled fields. 

<?php
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    foreach ($_POST as $key => $value) {
        if (empty($value)) {
            echo '<style>.highlight { border: 1px solid red; }</style>';
            break;
        }
    }
}
?>

<form method="post">
    <input type="text" name="name" class="<?php if($_SERVER["REQUEST_METHOD"] == "POST" && empty($_POST["name"])) { echo 'highlight'; } ?>">
    <input type="email" name="email" class="<?php if($_SERVER["REQUEST_METHOD"] == "POST" && empty($_POST["email"])) { echo 'highlight'; } ?>">
    <textarea name="message" class="<?php if($_SERVER["REQUEST_METHOD"] == "POST" && empty($_POST["message"])) { echo 'highlight'; } ?>"></textarea>
    <button type="submit">Submit</button>
</form>

Keywords

PHP form validation error handling highlighting user interface

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