PDO lastInsertId returns 0 - php

Question

I keep getting 0 in PDO lastInsertId. Here are my codes:
public static function createNewUser($email){
$password = self::rand_string(6);
$username = explode("#", $email);
$username = $username[0];
$passwordhashed = password_hash($password. self::salt(), PASSWORD_DEFAULT);
$register = self::connect()->prepare("INSERT INTO `user`(email,username,password,password_salt,type)VALUES(?,?,?,?,?)");
$register->execute(array($email,$username,$passwordhashed,$password,'customer'));
$user_id = self::pdolastid();
if($register){
return $user_id;
}
else{
return false;
}
}
and the last insert id function is:
public static function pdolastid(){
$last = self::connect()->lastInsertId();
return $last;
}
I have tried both with the pdolastid function and with self::connect()->lastInsertId().
The connect function is:
public static function connect(){
$servername = "localhost";
$username = "root";
$password = "";
try {
$conn = new \PDO("mysql:host=$servername;dbname=pw;port=3306;charset=utf8", $username, $password);
$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$conn->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
return $conn;
}
catch(PDOException $e)
{
$con_error = "Connection failed: " . $e->getMessage();
return $con_error;
}
}
The database has id as the primary key and is auto-increment
Someone please help