<?php

$abc = "Valdeir";
$abC = "Valdeir";

$abcd = [
    "Nome"   => "Valdeir",
    "github" => "https://g...content-available-to-author-only...b.com/valdeirpsr"
];

var_dump(
    isset($abc), //true
    isset($ABC), //false
    isset($aBc), //false
    isset($abC), //true

    isset($abcd['nome']),   //false
    isset($abcd['GitHub']), //false
    isset($abcd['Nome']),   //true
    isset($abcd['github'])  //true
);