Problem in passing value directly from browser URL

Dear All,

I am using Apache and PHP as local web server and now I am facing a problem.

I can directly run my php script on the web server, however, when I pass
argument or variable value directly from the URL address in browser, the PHP
script cannot determine it.

For example,
<?
    if ($a==1) {
        echo "True.";
    } else {
        echo "False.";
    }
?>

When I called the script http://localhost/show.php?a=1, the response is
"False.". Is there any problem in my script or the setting of my Apache
server is crashed?

Can anyone help me? Thank you.