formulaire avec redirection après validation

congratulation.php

<?php
echo "Merci à bientot";

form.php

<?php

$Firstname = $Lastname = $email = $comment = $number = "";
$FirstnameErr = $LastnameErr = $emailErr = $numberErr = "";
$err = 0;

if ($_SERVER["REQUEST_METHOD"] == "POST" && !empty($_POST)) {
    if (empty($_POST["Firstname"])) {
        $FirstnameErr = "Firstname is required";
        $err++;
    } else {
        $Firstname = test_input($_POST["Firstname"]);
        if (!preg_match("/^[a-zA-Z ]*$/", $Firstname)) {
            $FirstnameErr = "Only letters and white space allowed";
            $err++;
        }
    }


    if (empty($_POST["Lastname"])) {
        $LastnameErr = "Lastname is required";
        $err++;
    } else {
        $Lastname = test_input($_POST["Lastname"]);
        if (!preg_match("/^[a-zA-Z ]*$/", $LastnameErr)) {
            $LastnameErr = "Only letters and white space allowed";
            $err++;
        }
    }


    if (empty($_POST["email"])) {
        $emailErr = "Email is required";
        $err++;
    } else {
        $email = test_input($_POST["email"]);
        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $emailErr = "Invalid email format";
            $err++;
        }
    }


    if (empty($_POST["Number"])) {
        $numberErr = "Number is required";
        $err++;
    } else {
        $number = test_input($_POST["Number"]);
        if (!filter_var($number, FILTER_VALIDATE_REGEXP, array("options" => ['regexp' => '/[0-9]{10}/']))) {
            $numberErr = "Invalid format";
            $err++;
        }
    }


    if (empty($_POST["comment"])) {
        $comment = "";
    } else {
        $comment = test_input($_POST["comment"]);
    }

    if ($err == 0) {
        header('Location: congratulation.php');

    }
}


function test_input($data)
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}

?>


<form novalidate action="" method="post" class="sponsorship">
    <div class="sponsorship_div">
        <label for="Firstname" class="parrainage">Firstname</label>
        <input id="Firstname" required type="text" name="Firstname" class="mb-4 form-control">
        <span class="error"><?php echo $FirstnameErr; ?></span>
    </div>

    <div>
        <label for="Lastname" class="parrainage">Lastname</label>
        <input id="Lastname" required type="text" name="Lastname" class="mb-4 form-control">
        <span class="error"><?php echo $LastnameErr; ?></span>
    </div>

    <div>
        <label for="Number" class="parrainage">Number</label>
        <input id="Number" required type="tel" name="Number" pattern="[0-9]{10}" class="mb-4 form-control">
        <span class="error"><?php echo $numberErr; ?></span>
    </div>

    <div>
        <label for="email" class="parrainage">Write down your friend mail !</label>
        <input id="email" required type="email" name="email" placeholder="[email protected]" class="mb-4 form-control">
        <span class="error"><?php echo $emailErr; ?></span>
    </div>

    <div>
        <label for="select-option">Subject :</label>
        <select name="pickle" id="select-option" class="mb-4 form-control">
            <option value="">--Please choose an option--</option>
            <option value="A">A</option>
            <option value="B">B</option>
            <option value="C">C</option>
        </select>
    </div>


    <label class="parrainage">Send a funny note with your sponsorship link :-)</label>
    <div>
        <textarea name="comment" placeholder="Hey john ! This may interest you :D" class="mb-4 form-control"></textarea>
    </div>

    <div class="sponsorship_div">
        <div class="g-recaptcha" data-sitekey="6LfFCrkUAAAAAA2Fc-mvzCL5wiZKoRXcfLMKwYzc"></div>
        <br/>
        <input type="submit" value="Submit" class="w-50 btn btn-success">
    </div>
</form>