What is the difference between isset() and array_key_exists() in PHP? When to use which?

Hello everyone! I just started learning PHP and encountered a confusing situation. I have an array with user data: ```php $user = [ 'name' => 'Ivan', 'email' => 'ivan@example.com', 'age' => null, 'status' => '' ]; ``` I need to check if the key `age` exists in the array. I tried two ways: ```php if (isset($user['age'])) { echo "Age is set"; } if (array_key_exists('age', $user)) { echo "Key age exists"; } ``` The first method (`isset`) does not output a message, although the key `age` is clearly present in the array. But `array_key_exists` works and outputs "Key age exists". I'm confused: both seem to check for the existence of a key? Why different results? And for the `status` key with an empty string, `isset` also returns `false`, although