PDO: How to make checks - php

Question

I create this class but i'm newbie in PHP OOP & PDO and i don't know how and where i must to make check to username is valid , email is valid and e.t.c..
This is my code
Class Users {
private $db;
public function __construct(Database $datebase) {
if (!$database instanceOf Database) {
throw new Exeption();
}
$this->db = $datebase;
}
public function userRegistration($username, $password, $email) {
$username = $_POST['username'];
$password = $_POST['password'];
$email = $_POST['email'];
$regdate = date('d.m.Y');
$query = $this->db->prepare("INSERT INTO `users` (`username`, `password`, `email`, `regdate`) VALUES (?, ?, ?, ?) ");
$query->bindValue(1, $username);
$query->bindValue(2, $password);
$query->bindValue(3, $email);
$query->bindValue(4, $regdate);
return ($query->execute()) ? true : false ;
}
}

Answer 1

If you want to check something, use Respect/Validation. For example:
$usernameValidator = v::alnum()->noWhitespace()->length(1,15);
$usernameValidator->validate($_POST['username']); //true or false
$passwordValidator = v::alnum()->length(10, null);
$passwordValidator->validate($_POST['password']); //true or false
v::email()->validate($_POST['email']); //true or false

Answer 2

To check if the username or email exist in your database you can use SQL to search the email or username.
$query = $this->db->prepare("SELECT * FROM users WHERE email = ? ");
$query->bindValue(1, $email);
If the query returns a value than the email or username already exist in your database. From there you can show your own validation.

Answer 3

To check check if user or email exist you don't need another class, just add another method called userExist or emailExist and run a query and then check if you get a result.
public function emailExist($email){
$query = $this->db->prepare("SELECT * FROM users WHERE email = ? ");
$query->bindValue(1, $email);
try{
$query->execute();
//use the if statement and $query->rowCount() to check if there is a result
$rows = $query->rowCount();
if($rows === 1){
return true;
} else {
return false;
}
}catch (PDOException $e) {
die($e->getMessage());
}
}