Code:

<?php
$message="";

if(isset($_GET['pw']) && !empty($_GET['pw']))
{
  $message = "first message";
}
else if(isset($_GET['em']) && !empty($_GET['em']))
{
  $message = "second message";
}
else if(isset($_GET['fn']) && !empty($_GET['fn']))
{
  $message = "third message";
}
else if(isset($_GET['ln']) && !empty($_GET['ln']))
{
  $message = "forth message";
}
echo $message;
?>

Click to test

Result: