Access denied for user 'www-data'@'localhost' (using password: NO)

<?php
$servername = "localhost";
$username = "root";
$db_password = "";
$dbname = "accounts";

$conn = new mysqli($servername, $username, $db_password, $dbname);

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$email = $_POST['email'];
$password = $_POST['password'];

$password = md5($password);

$sql = "SELECT * from users WHERE email = '$email' AND password = '$password'";
$result = $conn->query($sql);
if ($result->num_rows == 0) {
  echo "<p>Email or password was entered incorrectly.</p>";
} else {
  echo "<p>Logged in successfully.</p>";

  $result = mysql_query("SELECT id,display_name,email FROM users WHERE email = '$email'");
  $row = mysql_fetch_row($result);

  echo $row[0];
  echo $row[1];
}
?>