PHP mysqli_query（）到PDO

I am trying to make a page to change a password in the database. I made the form and this is the PHP code :

if(isset($_POST['btn-newpass']))
{
$username = strip_tags($_POST['username']);
$password = md5(strip_tags($_POST['password']));
$password_new = md5(strip_tags($_POST['password_new']));
$password_new_conf = md5(strip_tags($_POST['password_new_conf']));
$password_in_db= mysqli_query("SELECT password FROM utilizatori WHERE username='$username'");

if(!$password_in_db)
{ echo "The entered username doesn't exist";}
elseif($password!=$password_in_db)
{ echo "The current password is wrong";}

if($password_new == $password_new_conf)
{$sql = mysqli_query("UPDATE utilizatori SET password='$password_new' WHERE username='$username'");}

if($sql)
{ echo "Changed successfully!";}
 else
{ echo "The passwords do not match";}
}

When I try to change a password I get the following errors:

Warning: mysqli_query() expects at least 2 parameters, 1 given in      A:\XAMPP\htdocs\testing\change_password.php on line 10
The entered username doesn't exist
Warning: mysqli_query() expects at least 2 parameters, 1 given in A:\XAMPP\htdocs\testing\change_password.php on line 18
Passwords do not match

In connection.php I have the following code:

class Database
{   
private $host = "localhost";
private $db_name = "atlx";
private $username = "root";
private $password = "";
public $conn;

public function dbConnection()
{

    $this->conn = null;    
    try
    {
        $this->conn = new PDO("mysql:host=" . $this->host . ";dbname=" . $this->db_name, $this->username, $this->password);
        $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);   
    }
    catch(PDOException $exception)
    {
        echo "Connection error: " . $exception->getMessage();
    }

    return $this->conn;
}
}

Could somebody point me out what is wrong here?

EDIT:

I realised the connection to the database is done using PDO. How can I convert the PHP code to work with PDO?

In the mysqli_query you have to pass the connection variable to for executing

Replace

$password_in_db= mysqli_query("SELECT password FROM utilizatori WHERE username='$username'")

With

$password_in_db= mysqli_query($con,"SELECT password FROM utilizatori WHERE username='".$username."'")

mysqli_query($Database->dbConnection(),"SELECT password FROM utilizatori WHERE username='$username'");

As mysqli_query expects parameter 1 to be connection object.

Replace

$password_in_db= mysqli_query("SELECT password FROM utilizatori WHERE username='$username'")

With

$conn = new Database;
$password_in_db= mysqli_query($conn->dbConnection(),"SELECT password FROM utilizatori WHERE username='".$username."'")

OR

$password_in_db= mysql_query("SELECT password FROM utilizatori WHERE username='".$username."'")

Since, You are using PDO for database connection why are you using mysqli_ to perform database query. Learn more about PDO

http://php.net/manual/en/book.pdo.php

on how to perform database query using it. OR learn how to make DB connection using mysqli

http://php.net/manual/en/book.mysqli.php

For your answer if you need to perform mysqli_query you need to add your Database connection in first parameter.

mysqli_query($dblink, "SELECT * FROM City")

Your Re-Written code should be like this

if(isset($_POST['btn-newpass']))
{
$username = strip_tags($_POST['username']);
$password = md5(strip_tags($_POST['password']));
$password_new = md5(strip_tags($_POST['password_new']));
$password_new_conf = md5(strip_tags($_POST['password_new_conf']));
$password_in_db= mysql_query("SELECT password FROM utilizatori WHERE username='".$username."'")

if(!$password_in_db)
{ echo "The entered username doesn't exist";}
elseif($password!=$password_in_db)
{ echo "The current password is wrong";}

if($password_new == $password_new_conf)
{$sql = mysqli_query("UPDATE utilizatori SET password='$password_new' WHERE username='$username'");}

if($sql)
{ echo "Changed successfully!";}
 else
{ echo "The passwords do not match";}
}

You can learn more about PDO in this link

http://php.net/manual/en/book.pdo.php