How to implement recursion in PHP?

Hello everyone! Please tell me how to properly implement recursion in PHP. I'm writing a function to traverse a multidimensional array and count all values, but I'm getting either 0 or an infinite loop.

Here's what I tried:

function recursiveSum($arr) {\n    static $sum = 0;\n    foreach ($arr as $value) {\n        if (is_array($value)) {\n            recursiveSum($value);\n        } else {\n            $sum += $value;\n        }\n    }\n    return $sum;\n}\n\n$data = [1, [2, 3], [4, [5, 6]]];\necho recursiveSum($data); // expected: 21

The problem is that when calling the function repeatedly with different arrays, the result accumulates due to the static variable. I also tried passing the sum by reference (&$sum), but then the function returns unexpected values. Also