Find all substrings within a string with overlap

Hi im trying to find all overlapping substrings in a string here is my code its only finding nonrepeating ACA.

$haystack = "ACAAGACACATGCCACATTGTCC";
$needle = "ACA";
echo preg_match_all("/$needle/", $haystack, $matches);

You're using echo to print the return value of preg_match_all. That is, you're displaying only the number of matches found. What you probably wanted to do was something like print_r($matches);, like this:

$haystack = "ACAAGACACATGCCACATTGTCC";
$needle = "ACA";
preg_match_all("/$needle/", $haystack, $matches);
print_r($matches);

Output:

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

)

Demo

If your real concern is that it counted ACACA only once, well, there are three things that need to be said about that:

1. That's basically unavoidable with regex.
2. You really shouldn't count this twice, as it's overlapping. It's not a true recurrence of the pattern.

3. That said, if you want to count that twice, you could do so with something like this:

   echo preg_match_all("/(?=$needle)/", $haystack, $matches);
   

   Output:

   4
   

   Demo