Try this UPDATED :: add a new line on each save:
$data=((string)$_REQUEST["data"]);
echo registration($data);
function registration($dat,$file = 'data.txt')
{
$result='';
$fileContent=file_get_contents($file);
if( strpos($fileContent,$dat) === false) {
$current = $fileContent;
$current .= "\r\n".$dat;
file_put_contents($file, $current);
$result.='Data Added ok!!';
} else {
$result.='Data Already Saved';
}
return $result;
}