i need an algorithm for comparing two strings for equivalence, but
levenshtein is just working for strings up to 255 characters, i have to
process longer strings (lets say, about 10kB).

does anybody know a *simple* algorithm, which brings me the distance like
levenshtein (maybe not so exact than this)?

what do You think about the cost function of levenshtein, and with it to
adjust the strings and use levensthein for each single partitioned string?

i need an algorithm for comparing two strings for equivalence, but
levenshtein is just working for strings up to 255 characters, i have to
process longer strings (lets say, about 10kB).

The Levenshtein algorithm has no limit in length. Your problem is probably
that PHP limits the length. Just implement the algorithm yourself. How it
works you can see e.g. on wikipedia: http://en.wikipedia.org/wiki/Levenshtein_distance