Why do we write the following code:

(isset{$_POST['posted']))

You mean isset($_POST['posted'])
The isset function is used to see if a variable has been declared, and is not NULL. If you try to use a variable which has not been declared PHP will generate a warning. See www.php.net/isset for a full description.

It is most often used to check that the input to the page ($_GET, $_POST, $_SESSION etc) has specific data set, for example if you have a page which is passed an ID in the querystring, you should check to see if the ID is present, so you can show your own error message or something.

<?php
if(isset($_GET['id']))
{
  $productid = $_GET['id'];
  // ... use the id to show details ...
}
else
{
  print "Invalid Product ID";
}
?>

Hope this helps
Phil