The combination of array intersect and unique pretty much solved both cases, then it was just using ord to convert char into ASCII int and subtracting correct amount to match the goal.
Only clever bit this day was in the second part to check for pairs that do not overlap and subtract that from number of all pairs.
Crate stacks so a just a simple array without need to use something fancy. Luckily I optimized first part to use array_splice instead of loop with array_pop and second part required just not reversing the spliced part.
Instead of going for any type of array I kept all processing in first part as a string using count_chars with mode 3 that returns string of unique characters if its length was 4 it meant all 4 were unique. It was perfect for the second part with minimal alternations.
The obvious solution would be to create tree, but I went with an associative array in which key was directory path. The traversing was very simple as I used array to keep current directory path adding to the end or popping last element to go level up. But then I needed to update size of each directory to contain its parent, and that again was done without recursive tree traversing but with double loop to check every directory against each other checking if the path contains one another i.e. path ./a should contain size of child directory ./a/b/c - all of that with simple strpos. Last tricky bit was adding missing directories that have not been initially in array of paths as task required to take any directory into considreation.
Like in day 7 I knew there will be a grid but how can I optimize traversing it. I don't need to check each cell in the grid individually because line of sight is shared in the whole row (or column) and each tree needs to be looked at from 4 directions. So instead of doing some weird calculation to loop each row and column in both orders I run over all trees 4 times with the same loop but after each iteration I use the array_map with null as a callback to rotate the grid by 90 deg. Part 2 can be done in the same loop just need additional foreach that check all following trees in current line of sight.