phpBB 3.x uses phpass
for hashing. However, we (unfortunately) use a custom hash identifier, which means that once you find the download link on that page (yeah, it's hard to find), you'll have to modify PasswordHash.php as follows:
In the `gensalt_private` method, change this line:
After that you should be able to use that `PasswordHash` to generate phpBB-compatible hashes!