<?php
ini_set('display_error', 1); // error display should come first
error_reporting(E_ALL); // error display should come first
require_once("database.php");

//define variables and initialize with empty values

    $firstnameError = $lastnameError = $emailError = $confirmemailError = "";
    $firstname = $lastname = $email = $confirmemail = '';

if (isset($_POST['register'])) { // $_POST['register'] instead


    if (empty($_POST['firstname'])) {
        $firstnameError = "field required";
    } else {
        $firstname = $_POST['firstname'];
    }

    if (empty($_POST['lastname'])) {
        $lastnameError = "field required";
    } else {
        $lastname = $_POST['lastname'];
    }

    if (empty($_POST['email'])) {
        $emailError = "field required";
    } else {
        $email = $_POST['email'];
    }

    if (empty($_POST['confirmemail'])) {
        $confirmemailError = "field required";
    } else {
        $confirmemail = $_POST['confirmemail'];
    }
}
?>

<form method="POST" action="">
    <div class="form-group">
        <label>First Name</label>
     <input type="text" class="form-control" placeholder="Enter first name" name="firstname" value="<?php echo htmlspecialchars($firstname);?>">
     <span class="error"><?php echo $firstnameError;?></span>
    </div>
    <div class="form-group">
        <label>Last Name</label>
        <input type="text" class="form-control" placeholder="enter last name" name="lastname" value="<?php echo htmlspecialchars($lastname);?>">
        <span class="error"><?php echo $lastnameError;?></span>
    </div>
    <div class="form-group">
        <label>Email</label>
        <input type="email" class="form-control" placeholder="enter email" name="email" value="<?php echo htmlspecialchars($email);?>">
        <span class="error"><?php echo $emailError;  ?></span>
    </div>
    <div class="form-group">
        <label>Confirm Email</label>
        <input type="email" class="form-control" placeholder="confirm email" name="confirmemail" value="<?php echo htmlspecialchars($confirmemail);?>">
        <span class="error"><?php echo $confirmemailError; ?></span>
    </div>
    <button type="submit" class="btn btn-default" name="register" value="submit">REGISTER</button> <!-- don't use camel case for form names -->
</form>