register.php

<?php
// Check whether a user with the given username exist or not
$query = "SELECT 1 FROM public.user WHERE username = :username";
$query_params = array(':username' => $username);
try{
    $stmt = $pdo->prepare($query);
    $result = $stmt->execute($query_params);
}
catch(PDOException $ex){
    echo '<span style="color: red">ERROR! Code: 009</span>';
    exit;
}

$row = $stmt->fetch();

// When a user exist with the username, registration fails
if($row){
    $error = 1;
}

// If no error has occurred so far, create the user
if($error != 1) {
    // If the register as administrator checkbox was activated in the registration form, create an administrator account, when not create a normal account
    if($_POST['isAdmin'] == 'on'){
        $query = "INSERT INTO public.user (username, unencrypted_password, is_admin) VALUES (:username, :unencrypted_password, TRUE)";
    }else{
        $query = "INSERT INTO public.user (username, unencrypted_password, is_admin) VALUES (:username, :unencrypted_password, FALSE)";
    }
    $query_params = array(':username' => $username, ':unencrypted_password' => $password);
    try{
        $stmt = $pdo->prepare($query);
        $result = $stmt->execute($query_params);
    }
    catch(PDOException $ex){
        echo '<span style="color: red">ERROR! Code: 010</span>';
        exit;
    }
}
?>