Mysql error message using PDO - mysql

Question

I'd like my database not to insert duplicate e-mails, I used the unique key and it works perfectly. However, no message is shown to the user, and it seems like he signed up correctly, but when he tries to login he finds out it's impossible.
I made this code, but it returns success for both situations.
$sql4 = "INSERT INTO users (username, email, password, id_group, id_company)
VALUES (:username, :email, :password, :id_group, :id_company)";
$stmt4 = $PDO->prepare($sql4);
$stmt4->bindParam(':username', $username);
$stmt4->bindParam(':email', $email);
$stmt4->bindParam(':password',md5($password));
$stmt4->bindParam(':id_group', $id_group);
$stmt4->bindParam(':id_company', $id_company);
$stmt4->execute();
$erro = mysql_errno();
if ($erro == 0) {
echo '<p align="center">Success!</p>';
} elseif ($erro == 1062) {
echo '<p align="center"> Email already registered. Try using another.
</p>';
}

Answer 1

I tried another way and it worked:
$sql4 = $PDO->prepare("SELECT COUNT(*) as us FROM users WHERE email =
:email");
$sql4->bindValue(":email",$email);
$sql4->execute();
$row = $sql4->fetch();
if($row['us'] == '0'){
$sql4 = "INSERT INTO users (username, email, password, id_group, id_company)
VALUES (:username, :email, :password, :id_group, :id_company)";
$stmt4 = $PDO->prepare($sql4);
$stmt4->bindParam(':username', $username);
$stmt4->bindParam(':email', $email);
$stmt4->bindParam(':password',md5($password));
$stmt4->bindParam(':id_group', $id_group);
$stmt4->bindParam(':id_company', $id_company);
$stmt4->execute();
return '1';
//se o email for repetido exibe mensagem de erro.
}else{
echo 'Email already registered. Try another.';
}