PHP script does not understand the variables from the URL?

Hello, can you please help me with the following very strange problem? It is the first time I ever see a problem like this: the PHP script does not understand the variables passed to it by URL. To explain the point I made a very simple script test.php consisting of the text:

<?php
echo $signal;
?>

So if we call this script by browser as:
www.MyServer.com/test.php?signal=777
then it need output the text “777”. However, it does not do that.

To test if PHP in general works on this server I made another simple script like:

<?php
$signal = 888;
echo $signal;
?>

Which DID output the text “888”

Can somebody have an idea what is the problem?

Thank you!