But in O notation it will be 3x vs x² i think this is still better than having nested maps or foreach.

I think the issue is fixed, could you please verify?

It seems to be a bug, I'm not sure yet if it's a bug in the kata, or in setup of PHP on Codewars.

No, neither of those two is correct, and at least in PHP (you didn't mention the language), there is no test with "[[(]])" as input.

Please read the note:

Looping all the way up to n, or n/2, will be too slow.

And that's exactly what your code is doing. Not a kata issue.

Kata is perfectly solvable in PHP:

PHP Completions 2301

And the highest tested value for PHP is 9294972289. It doesn't say there are 2^31 tests, it says for some languages there can be tests where n is near 2^31.

not an issue, PHP can be solved in 1 second.