gistfile2.txt

PHP Notice:  Undefined index: invalid in .../bah.php on line 4
PHP Stack trace:
PHP   1. {main}() .../bah.php:0

test.php

<?php

$array = array();
echo @$array['invalid'];
echo "\n";

if (isset($array, 'invalid')) {
    echo $array['invalid'];
    echo "\n";
}