browser gives 500 error code

I have written this php code to execute after a form is submitted. When the form is submitted the browser just gives an error code 500, server error. Not sure what I'm doing wrong here. I tried to add try-catch to find out what this error was but it doesn't help.

<?php
    try {
        $firstname = htmlspecialchars($_POST['firstname']);
        $lastname = htmlspecialchars($_POST['lastname']);
        $email = htmlspecialchars($_POST['email']);
        $phone = htmlspecialchars($_POST['phone']);

        #
        $firstname = mysqli_real_escape_string($firstname);
        $lastname = mysqli_real_escape_string($lastname);
        $email = mysqli_real_escape_string($email);
        $phone = mysqli_real_escape_string($phone);

        $servername = "localhost";
        $username = "airsofto_oliver";
        $password = "magnetar1";
        $dbname = "airsofto_YummyYummy";

        // Create connection
        $conn = new mysqli($servername, $username, $password, $dbname);
        // Check connection
        if ($conn - > connect_error) {
            die("Connection failed: ".$conn - > connect_error);
        }

        $sql = "INSERT INTO `airsofto_YummyYummy`.`Customers` (`CustomerID`, `FirstName`, `LastName`, `Email`, `Tel`, `AddressFirstLine`, `AddressSecondLine`, `Postcode`, `Timestamp`) VALUES (NULL, '$firstname', '$lastname', '$email', '$phone', '', '', '', CURRENT_TIMESTAMP);";

        if ($conn - > query($sql) === TRUE) {
            echo "New record created successfully";
        } else {
            echo "Error: ".$sql.
            "<br>".$conn - > error;
        }

        $conn - > close();
    } catch (Exception $e) {
        echo 'Caught exception: ', $e - > getMessage(), "\n";
    }
?>

Thanks in advance for everyone's help.