How to get the first key in a PHP associative array (without resetting the pointer)?

Hello everyone! I'm a beginner PHP developer and I'm having an issue with arrays.

I have an associative array that can be either empty or contain data. I need to get only the first key of this array without modifying its internal pointer. The problem is that I don't know in advance what keys the array will have — they can be either numeric or string.

Here's an example of my code:

$users = [
    'john' => ['name' => 'John', 'age' => 30],
    'jane' => ['name' => 'Jane', 'age' => 25],
    'bob'  => ['name' => 'Bob', 'age' => 35]
];

// I need to get the first key 'john'
$firstKey = ???

I've tried using the key(), reset(), and array_keys() functions, but