php将变量作为键获取,没有赋值

<?php
$key = 'name';
$array = [
    'name' => 'John',
    'age' => 30,
    'city' => 'New York'
];

$value = $array[$key];
echo $value; // 输出 "John"
?>

<?php
$key = null;
$array = [
    'name' => 'John',
    'age' => 30,
    'city' => 'New York'
];

$value = $array[$key];
echo $value; // 输出 ""
?>