$id = 1;
$age = 2;
$sort = "id"; // or "age";
$Key = $$sort;
$arr = array($Key => 'string');
print_r($arr);
$array = array(0 => 100, "cor" => "vermelho");
print_r(array_keys($array));
$array = array("azul", "vermelho", "verde", "azul", "azul");
print_r(array_keys($array, "azul"));
$array = array("cor" => array("azul", "vermelho", "verde"),
"tamanho" => array("pequeno", "medio", "grande"));
print_r(array_keys($array));