include_once gives an error when variable isn't passed in the url
    index.php
    <?php include_once 'lang.php'; ?>

    lang.php
    <?php
    session_start();
    header('Cache-control: private'); // IE 6 FIX
    if(isSet($_GET['lang']))
    {
    $lang = $_GET['lang'];
    // register the session and set the cookie
    $_SESSION['lang'] = $lang;
    setcookie("lang", $lang, time() + (3600 * 24 * 30));
    }
    else if(isSet($_SESSION['lang']))
    {
    $lang = $_SESSION['lang'];
    }
    else if(isSet($_COOKIE['lang']))
    {
    $lang = $_COOKIE['lang'];
    }
    else
    {
    $lang = 'en';
    }
    switch ($lang) {
    case 'en':
    $lang_file = 'lang.en.php';
    break;
    case 'pl':
    $lang_file = 'lang.pl.php';
    break;
    }
    include_once 'languages/'.$lang_file;
    ?>

In lang.en.php and lang.pl.php I have arrays with content. 

When website is opened for the first time I get two warnings:

Warning: include_once(languages/) [function.include-once]: failed to open stream: No such file or directory in blah/blah/blah/lang.php on line 37

Warning: include_once() [function.include]: Failed opening 'languages/' for inclusion (include_path='.:/usr/local/lib/php-5.2.17/lib/php') in blah/blah/blah/lang.php on line 37

Then when I select a language on the webpage, and variable "lang" is passed in the url (e.g. index.php?lang=en) everything works ok. How to fix it?
include_once gives an error when variable isn't passed in the url
===

    index.php
    <?php include_once 'lang.php'; ?>

    lang.php
    <?php
    session_start();
    header('Cache-control: private'); // IE 6 FIX
    if(isSet($_GET['lang']))
    {
    $lang = $_GET['lang'];
    // register the session and set the cookie
    $_SESSION['lang'] = $lang;
    setcookie("lang", $lang, time() + (3600 * 24 * 30));
    }
    else if(isSet($_SESSION['lang']))
    {
    $lang = $_SESSION['lang'];
    }
    else if(isSet($_COOKIE['lang']))
    {
    $lang = $_COOKIE['lang'];
    }
    else
    {
    $lang = 'en';
    }
    switch ($lang) {
    case 'en':
    $lang_file = 'lang.en.php';
    break;
    case 'pl':
    $lang_file = 'lang.pl.php';
    break;
    }
    include_once 'languages/'.$lang_file;
    ?>

In lang.en.php and lang.pl.php I have arrays with content. 

When website is opened for the first time I get two warnings:

Warning: include_once(languages/) [function.include-once]: failed to open stream: No such file or directory in blah/blah/blah/lang.php on line 37

Warning: include_once() [function.include]: Failed opening 'languages/' for inclusion (include_path='.:/usr/local/lib/php-5.2.17/lib/php') in blah/blah/blah/lang.php on line 37

Then when I select a language on the webpage, and variable "lang" is passed in the url (e.g. index.php?lang=en) everything works ok. How to fix it?