To concatenate two string using fwrite()

{php}
echo "{a href='sampl.php?id=hai'}Click Here{/a}";
if(isset($_REQUEST['id']))
{
$fp = fopen('data.txt', 'a');
//fwrite($fp, );
fwrite($fp, $_REQUEST['id']);
fclose($fp);


echo"authorize";
}
else
echo"Unauthorize";
{/php}