Get output of a PHP script executed in Bash

Question

I made a bash script for rebuilding my satis repository. I want to add some logging, because the command will be run automatically.

When I execute

output="$(php -f vendor/bin/satis build \
  --repository-url git@bitbucket.org:example/gibberish.git satis.json web/ -n)"

$output should be

[InvalidArgumentException] Specified repository url "git@bitbucket.org:example/gibberish.git" does not exist.

build [--repository-url [REPOSITORY-URL]] [--no-html-output] [--skip-errors] [--stats] [--] [] [] []...

However, I only get Scanning packages How do I catch an exception thrown by a PHP script in Bash?