Error with variable

[thriller]

I need to check if a variable has a value. This works great, but when it doesn't have a value I get this error.

---------- code --------------------
Notice: Undefined index: id in c:\inetpub\wwwroot\php\index.php on line 13
---------- // code --------------------

Should I define the variable before using it, if so how is that done.

What else can I do about this. I use this code to check the variable

---------- code --------------------
if ($REQUEST["id"] != "") {
$session_id = $REQUEST["id"];
} else {
$session_id = "";
}
---------- // code --------------------

[drawmack]

if (isset($_REQUEST["id"]) && $_REQUEST["id"] != "") {
  $session_id = $_REQUEST["id"];
} else {
  $session_id = "";
}

or

if (array_key_exists('id',$_REQUEST) && $_REQUEST["id"] != "") {
  $session_id = $_REQUEST["id"];
} else {
  $session_id = "";
}

PHP is short circuit technology so if the first function returns false then the second one will not execute and the error will not show up.

Here is where you can find more information on these two functions:
http://us4.php.net/manual/en/function.array-key-exists.php
http://us4.php.net/manual/en/function.isset.php