$_GET is not working

<?php 
include 'core/init.php';
logged_in_redirect();
include 'includes/overall/header.php';

if (empty($_POST) === false) {
    $required_fields = array('first_name', 'last_name', 'username', 'password', 'password_again', 'email');
    foreach($_POST as $key=>$value) {
        if (empty($value) && in_array($key, $required_fields) === true) {
            $errors[] = 'All field are required';
            break 1;
        }
    }

    if (empty($errors) === true) {
        if (user_exists($_POST['username']) === true) {
            $errors[] = 'Sorry, the user \'' . $_POST['username'] . '\' is already taken';
        }
        if (preg_match("/\\s/", $_POST['username']) == true) {
            $errors[] = 'Your username can not have spaces';

        }
        if (strlen($_POST['password']) < 6) {
            $errors[] = 'Your password must be at least 6 characters long';
        }

        if ($_POST['password'] !== $_POST['password_again']) {
            $errors[] = 'Your passwords do not match';
        }

        if (filter_var($_POST['email'], FILTER_VALIDATE_EMAIL) === false) {
            $errors[] = 'A valid email address is required';
        }

        if (email_exists($_POST['email']) === true) {
            $errors[] = 'Sorry, the email \'' . $_POST['email'] . '\' is already is use. Please contact your site adimn is this is incorrect.';
        }

    }
}
 ?>

    <div class="container background">
        <?php
        if (isset($_GET['success']) && empty($_GET['success'])) {
            echo 'You\'ve be registered successfully! Please check your email to activate your account';
        } else {
            if(empty($_POST) === false && empty($errors) === true) {
                $register_data = array(
                    'first_name'    => $_POST['first_name'],
                    'last_name'     => $_POST['last_name'],
                    'username'  => $_POST['username'],
                    'password'      => $_POST['password'],
                    'email'         => $_POST['email'],
                    'email_code'    => md5($_POST['username'] + microtime())
                );

                register_user($register_data);
                header('Location: register.php?success');
                exit();

            } else if (empty($errors) === false) {
                echo output_errors($errors);
            }

        ?>
            <form action="" method="POST" class="form-horizontal" role="form">
              <div class="form-group">
                <label for="inputfirstname3" class="col-sm-2 control-label">First Name</label>
                <div class="col-sm-10">
                  <input type="text" name="first_name" class="form-control" id="inputfirstname3" value="" autocomplete="off" placeholder="First Name">
                </div>
              </div>
              <div class="form-group">
                <label for="inputlastname3" class="col-sm-2 control-label">Last Name</label>
                <div class="col-sm-10">
                  <input type="text" name="last_name" class="form-control" id="inputlastname3" value="" autocomplete="off" placeholder="Last Name">
                </div>
              </div>
              <div class="form-group">
                <label for="inputusername3" class="col-sm-2 control-label">Username</label>
                <div class="col-sm-10">
                  <input type="text" name="username" class="form-control" id="inputusername3" value="" autocomplete="off" placeholder="Username">
                </div>
              </div>
              <div class="form-group">
                <label for="inputemail3" class="col-sm-2 control-label">Email</label>
                <div class="col-sm-10">
                  <input type="email" name="email" class="form-control" id="inputemail3" value="" autocomplete="off" placeholder="Email Address">
                </div>
              </div>
              <div class="form-group">
                <label for="inputpassword3" class="col-sm-2 control-label">Password</label>
                <div class="col-sm-10">
                  <input type="password" name="password" class="form-control" id="inputpassword3"  autocomplete="off" placeholder="Password">
                </div>
              </div>
              <div class="form-group">
                <label for="inputpassword_again3" class="col-sm-2 control-label">Validate Password</label>
                <div class="col-sm-10">
                  <input type="password" name="password_again" class="form-control" id="inputpassword_again3" autocomplete="off" placeholder="Validate Password">
                </div>
              </div>
              <div class="form-group">
                <div class="col-sm-10">
                  <p class="info">Please be aware that this does not mean that you are a member of Better Gamerz United. If you would like to be come a member please
                  contuine with the registration and then fill out our application. Other wise you will have limited use of this site. Thank you Team BGU!! </p>
                </div>
              </div>
              <div class="form-group">
                <div class="col-sm-offset-2 col-sm-10">
                    <button type="submit" class="btn btn-primary pull-right register">Register</button>
                </div>
              </div>
            </form>
        <?php } ?>
    </div>
<?php include 'includes/overall/footer.php'; ?>