Somehow can't prevent name duplication - mysql

Question

I recently ran into some trouble with my registration system, I've tried to prevent name duplication in a lot of ways, And already browsed stackoverflow aswell.
But i can't seem to find the solution.
Here is my code
<?php
if (isset($_POST['registerBtn']))
{
$username = $_POST['username'];
$password = $_POST['password'];
$rpassword = $_POST['rpassword'];
$email = $_POST['email'];
$errors = array();
$checkUsername = $odb -> prepare("SELECT COUNT(*) as total from FROM `users` WHERE `username` = :username");
$checkUsername -> execute(array(':username' => $username));
$countUsername = $checkUsername -> fetchColumn(0);
if ($checkUsername > 0)
{
$errors['Username is already taken'];
}
if (!ctype_alnum($username) || strlen($username) < 4 || strlen($username) > 15)
{
$errors[] = 'Username Must Be Alphanumberic And 4-15 characters in length';
}
if (!filter_var($email, FILTER_VALIDATE_EMAIL))
{
$errors[] = 'Email is invalid';
}
if (empty($username) || empty($password) || empty($rpassword) || empty($email))
{
$errors[] = 'Please fill in all fields';
}
if ($password != $rpassword)
{
$errors[] = 'Passwords do not match';
}
if (empty($errors))
{
$insertUser = $odb -> prepare("INSERT INTO `users` VALUES(NULL, :username, :password, :email, 0, 0, 0, 0)");
$insertUser -> execute(array(':username' => $username, ':password' => SHA1($password), ':email' => $email));
echo '<div class="nNote nSuccess hideit"><p><strong>SUCCESS: </strong>User has been registered. Redirecting....</p></div><meta http-equiv="refresh" content="3;url=login.php">';
}
else
{
echo '<div class="nNote nFailure hideit"><p><strong>ERROR:</strong><br />';
foreach($errors as $error)
{
echo '-'.$error.'<br />';
}
echo '</div>';
}
}
?>

Answer 1

You can put a UNIQUE constraint on username as such:
CREATE TABLE users
(
id int NOT NULL AUTO_INCREMENT,
username varchar(30),
password varchar(80),
email varchar(80),
PRIMARY KEY (id),
UNIQUE KEY username (username)
)

Answer 2

You should be comparing to $countUsername, not $checkUsername.
if ($checkUsername > 0)
Change to
if ($countUsername> 0)
Also, this line should be changed from:
if ($checkUsername > 0)
{
$errors['Username is already taken'];
}
To
if ($checkUsername > 0)
{
$errors[] = 'Username is already taken';
}
See below:
<?php
if (isset($_POST['registerBtn']))
{
$username = $_POST['username'];
$password = $_POST['password'];
$rpassword = $_POST['rpassword'];
$email = $_POST['email'];
$errors = array();
$checkUsername = $odb -> prepare("SELECT COUNT(*) as total from FROM `users` WHERE `username` = :username");
$checkUsername -> execute(array(':username' => $username));
$countUsername = $checkUsername -> fetchColumn(0);
$errors = [];
if ($countUsername > 0)
{
$errors[] = 'Username is already taken';
}
if (!ctype_alnum($username) || strlen($username) < 4 || strlen($username) > 15)
{
$errors[] = 'Username Must Be Alphanumberic And 4-15 characters in length';
}
if (!filter_var($email, FILTER_VALIDATE_EMAIL))
{
$errors[] = 'Email is invalid';
}
if (empty($username) || empty($password) || empty($rpassword) || empty($email))
{
$errors[] = 'Please fill in all fields';
}
if ($password != $rpassword)
{
$errors[] = 'Passwords do not match';
}
if (count($errors) > 0)
{
$insertUser = $odb -> prepare("INSERT INTO `users` VALUES(NULL, :username, :password, :email, 0, 0, 0, 0)");
$insertUser -> execute(array(':username' => $username, ':password' => SHA1($password), ':email' => $email));
echo '<div class="nNote nSuccess hideit"><p><strong>SUCCESS: </strong>User has been registered. Redirecting....</p></div><meta http-equiv="refresh" content="3;url=login.php">';
}
else
{
echo '<div class="nNote nFailure hideit"><p><strong>ERROR:</strong><br />';
foreach($errors as $error)
{
echo '-'.$error.'<br />';
}
echo '</div>';
}
}
?>