<?php
//SOMETHING = NOTHING
$something = '';
echo "\$something = '$something';\n";
if (!isset($something))
{
    echo 'Error <- Your code';
}
else
{
    echo 'Success <- Your code';
}

if (!isset($something) || $something == '')
{
    echo "\nError\n\n";
}
else
{
    echo "\nSuccess\n\n";
}

//SOMETHING = 0
$something = '0';
echo "\$something = '$something';\n";
if (!isset($something))
{
    echo 'Error <- Your code';
}
else
{
    echo 'Success <- Your code';
}

if (!isset($something) || $something == '')
{
    echo "\nError\n\n";
}
else
{
    echo "\nSuccess\n\n";
}
?>

$something = '';
Success <- Your code
Error

$something = '0';
Success <- Your code
Success