如何确定关联数组是否有键？

$array = [
    'key1' => 'value1',
    'key2' => 'value2',
];

if (array_key_exists('key1', $array)) {
    echo 'Key1 exists in the array.';
} else {
    echo 'Key1 does not exist in the array.';
}

$array = [
    'key1' => 'value1',
    'key2' => 'value2',
];

if (isset($array['key1'])) {
    echo 'Key1 exists in the array.';
} else {
    echo 'Key1 does not exist in the array.';
}