As you can see, I've been through some hell this morning. First of all, you're only going to get presented with:

In Process.php line 441:

  The process has been signaled with signal "6".

...if there is a composer.json file in the current working directory, so if you run composer right now in, say, $HOME you're going to think it's working, but if you're here you likely have figured this out...

I did so much googling this morning. But I eventually landed on this, which gave me a clue...

Running composer -vvv I saw the same thing, and running svn --version I was presented with a new error:

dyld[92827]: Symbol not found: _apr_crypto_block_cleanup
  Referenced from: /opt/homebrew/Cellar/subversion/1.14.2/lib/libsvn_subr-1.0.dylib
  Expected in: /usr/lib/libaprutil-1.0.dylib

So I reinstalled svn, php, and composer a bazillion times, no effect. But then I landed on this in my googling and figured, what the hell and ran the suggested fix on that seemingly unrelated issue:

brew reinstall apr-util

And...it worked! I don't really know what apr-util is, but it seems to have at least fixed my problem for now, and hopefully yours!