<?php

$strings = ["folders/folder1/folder5/../../folder2", "folders/folder1/../../../folder2", "folders/folder1/folder5/../folder2"];

function make_path($string) {
	$parts = explode("/", $string);
	$new_folder = [];
	for ($i=0; $i<count($parts); $i++) {
		if (($parts[$i] == "..") and count($new_folder) >= 1) {
			array_pop($new_folder);
		} else {
			$new_folder[] = $parts[$i];
		}
	}
	return implode("/", $new_folder);
}

$new_folders = array_map('make_path', $strings);
print_r($new_folders);
?>