login with username or email address in php
$username=$_REQUEST['login'];
$email=$_REQUEST['login'];
$password=$_REQUEST['password'];

if($username && $password) {
  $query="select * from  user_db where username='$username'  and password='$password'";
} else if ($email && $password) {
  $query="select * from  user_db where email='$email' and password='$password'";
}

$login=$_REQUEST['login'];
$query = "select * from  user_db where ( username='$login' OR email = '$login') and password='$password'"

$username=$_REQUEST['login'];
$email=$_REQUEST['login'];

$username=$_REQUEST['username'];//I'm assuming your code here was wrong
$email=$_REQUEST['email'];//and that you have three different fields in your form
$password=$_REQUEST['password'];

if (validate_username($username)) {
  $query="select * from  user_db where username='".$username".' and password='".validate_password($password)."'";
} else if (validate_email($email)) {
  $query="select * from  user_db where email='".$email."' and password='".validate_password($password)."'";
}

//... elsewhere...

function validate_username(&$username) {
  if (strlen($username) <= 1) { return false; }
  //return false for other situations
    //Does the username have invalid characters?
    //Is the username a sql injection attack?
  //otherwise...
  return true;
}

function validate_email(&$email) {
  //same deal as with username
}

function validate_password(&$password) {
  //same deal as with username
}