Как исправить ошибки Notice?

Question

[s_katala]

Notice: Undefined index: act in 
Notice: Undefined index: lang_list in 
Notice: Undefined index: lang in 
Notice: Undefined variable: checkLang in

if($_GET['act'] == 'chage_lang'){
	$langId = intval($_GET['id']);
	$config['lang_list'] = nl2br($config['lang_list']);
	$expLangList = explode('<br />', $config['lang_list']);
	$numLangs = count($expLangList);
	if($langId > 0 AND $langId <= $numLangs){
		set_cookie("lang", $langId, 365);
	}
	$langReferer = $_SERVER['HTTP_REFERER'];
	header("Location: {$langReferer}");
}
$config['lang_list'] = nl2br($config['lang_list']);
$expLangList = explode('<br />', $config['lang_list']);
$numLangs = count($expLangList);
$useLang = intval($_COOKIE['lang']);
if($useLang <= 0) $useLang = 1;
$cil = 0;
foreach($expLangList as $expLangData){
	$cil++;
	$expLangName = explode(' | ', $expLangData);
	if($cil == $useLang AND $expLangName[0]){
		$rMyLang = $expLangName[0];
		$checkLang = $expLangName[1];
	}
}
if(!$checkLang){
	$rMyLang = 'русский';
	$checkLang = 'russian';
}