PHP Undefined variable: mail in Hatası

Hata veren satır mail($mail,'=?utf-8?B?'.base64_encode($subject).'?=',$txt,$headers);

{

$headers  = "MIME-Version: 1.0n";
$headers .= "Content-type: text/html; charset=UTF-8n";
$headers .= "X-Mailer: PHPn";
$headers .= "X-Sender: PHPn";


$subject = "".$demo[1][$i]."";
$txt = "".$title[1][$i]."<br>".$title[1][$i]."<br>";


mail($mail,'=?utf-8?B?'.base64_encode($subject).'?=',$txt,$headers);

echo 'İşlem onaylandı: '.$title[1][$i];
echo '<br>';
}