I am trying to find offset of all occurrences with preg_match_all

e.g.

$haystack = 'aaaab';
$needle = 'aa';

preg_match_all('/' . $needle . '/', $haystack, $matches);

$matches is

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [0] => aa
                    [1] => 0
                )

            [1] => Array
                (
                    [0] => aa
                    [1] => 2
                )

        )

)

It returns offset of first and second group of aa ("aa" "aa" "b") from the haystack, while I am expecting it to return "aa" starting at index 1 as well.

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [0] => aa
                    [1] => 0
                )

            [1] => Array
                (
                    [0] => aa
                    [1] => 1
                )

            [2] => Array
                (
                    [0] => aa
                    [1] => 2
                )

        )

)

Is there a way I can fix the regex or use some other function (which accepts regex) to get this done?

PS: I know strpos which can do this, but I have few more things to search for hence will go with preg_match_all.