the find method return value

I have a problem when I using the method find in models.

if I have a table called user like this :

id name

1 linbob

2 phalcon

3 framework

My code:

$userData = User::find();

foreach($userData as $key => $data) {

 echo 'key is '.$key.' and user is '.$data->name.'<br />';

}

the output is :

linbob

phalcon

framework

but if I have the code :

$userData = User::find();

echo $userData[-1]->name;

echo $userData[-100]->name;

why the output is the first value and why this key has value?