<?php

$str = '/category1/category2/page.html';

$parts = preg_split('/(?<=\w\/)(?=\w)/', $str, -1, PREG_SPLIT_NO_EMPTY);
for ($i = 1; $i < sizeof($parts); $i++) {
	$parts[$i] = $parts[$i - 1] . $parts[$i];
}

var_dump($parts);