I can't find one and I don't know if any of PHP Compression and Archive Extensions will work.
Do you think I could use a compression stream to read data from a 7z file?
UPDATE
The 7z file format can use various compression algorithms, so you might be able to decompress the archive with one of the existing utilities for bzip2 or deflate.
I found a 7z PHP class as well, and you are lucky since it's still being developed. Here is the latest version.