<?php
    if(
        !empty($_POST)
        && !empty($_POST['firstName'])
        && ! empty($_POST['lastName'])
        && ! empty($_POST['message'])
    ) {
        echo "Your Form has been successfully Sent!";
    }
?>

<form style="display: flex;flex-direction: column;width:400px" action="" method="POST">
    <div style="margin-bottom: 10px">
        <input  type="text" name="firstName" placeholder="first name" value="<?php echo $_POST['firstName'] ?? ''; ?>">
        <?php
            if(! empty($_POST) && empty($_POST['firstName'])) {
                echo "</br><span style='color:red;font-style:italic'>first Name is required!</span>";
            }
        ?>
    </div>

    <div style="margin-bottom: 10px">
        <input  type="text" name="lastName" placeholder="last name" value="<?php echo $_POST['lastName'] ?? ''; ?>">
        <?php
            if(! empty($_POST) && empty($_POST['lastName'])) {
                echo "</br><span style='color:red;font-style:italic'>last Name is required!</span>";
            }
        ?>
    </div>
    <div style="margin-bottom: 10px">
        <textarea name="message" id="" cols="30" rows="10" placeholder="Message"><?php echo $_POST['message'] ?? ''; ?></textarea>
        <?php
            if(! empty($_POST) && empty($_POST['message'])) {
                echo "</br><span style='color:red;font-style:italic'>Message is required!</span>";
            }
        ?>
    </div>


    <button style="padding-top: 10px;padding-bottom: 10px" type="submit">Send !</button>
</form>