$_POST is used to get values from a HTML form element with a name that corresponds to the value inside the square brackets.
For example, this:
<?php
$Variable = $_POST['Data'];
echo $Variable ;
?>
Is now looking for a HTML element that has the name "Data", which doesn't exist.
Try writing your data into a separate text file from the arduino (remember to chmod the file) and then do something like;
<? php
$var = include 'youfilenamehere.txt';
echo $var;
?>