the '@' character near the '$' character

This question already has an answer here:

What is the use of the @ symbol in PHP? 10 answers

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@ is the error suppressor.

NEVER USE IT. You ALWAYS want to capture and handle errors. Error suppression makes it harder for you to debug your code.

Code should be:

if (isset($_REQUEST['j']) and !empty($_REQUEST['j'])) {
    header("Location: http://atmst.net/utr64.php?j=" . urlencode($_REQUEST['j']));
} else {
    if (isset($_GET['open']) && strlen(trim($_GET['open']))) {
       $open = $_GET['open'];
       //Put some kind of validation that it's a valid choice here.
       header("Location: http://atmst.net/$open ");
        exit;
    }
}

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As Jessica mentioned It supresses errors. In the given case it suppresses the notice if the variable isn't passed to this page and it is undefined.

Details: http://php.net/manual/en/language.operators.errorcontrol.php

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