Drop Down Select File And Include It Into Same Page

Tried to do this but it's only echoing the value not actually choosing the .php file to include into the same page. The code is below...

]/$file.php;

to

$path_file = $_SERVER['DOCUMENT_ROOT']."/".$file.".php";

But this is very bad for security (you can't trust _POST data). Better change to something like:

if (isset($_POST['select']) && $_POST['select'])
{
if ($_POST['select'] == 'something')
 include'something.php';
if ($_POST['select'] == 'somethingelse')
 include'another.php';
} else {
  include'default.php';
}