This is PHP Composer, debianized.
This is packaged on Launchpad, at https://launchpad.net/composer.
Add the PPA:
apt-add-repository -y ppa:duggan/composer
apt-get update
Then install with:
apt-get install php5-composer
Releases are tagged with a date, as the composer project itself does not hold to any useful versioning mechanism.
The PPA is set up for daily builds, though I expect this to settle into a roughly weekly cadence of release.
To update:
apt-get update
apt-get install --only-upgrade php5-composer