PHP arrays are weird. So the result of

<?php
$array = [
    'element0',
    'element1' => 'Hello World',
    'element2' => 42,
    'element3' => [ 'aaa', 'bbb' ],
];
print_r( array_keys($array) );
?>
is a surprising though correct
Array
(
    [0] => 0
    [1] => element1
    [2] => element2
    [3] => element3
)

Arrays in php are really ordered key value pairs where if a value is missing it will become a value with a number as key. To get the intuitive first dimension of an array use a map with function to determine if a key is in fact an index.
This

<?php
$array_first = array_map(
    fn ($key)=>
        is_int($key)?$array[$key]:$key
    ,
    array_keys($array)
);
print_r($array_first);
?>

will show

Array
(
    [0] => element0
    [1] => element1
    [2] => element2
    [3] => element3
)

However in a niche case 
<?php
$array = [
    'element0',
    'element1' => 'Hello World',
    'element2' => 42,
    'element3' => [ 'aaa', 'bbb' ],
    12 => 'I\'m a field, not a key'
];
?>
this won't work of course and output
Array
(
    [0] => element0
    [1] => element1
    [2] => element2
    [3] => element3
    [4] => I'm a field, not a key
)