$ php composer.phar require sabre/vobject "^4.0"
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.
Problem 1
- The requested package sabre\vobject could not be found, it looks like its name is invalid, "\" is not allowed in package names.
Potential causes:
- A typo in the package name
- The package is not available in a stable-enough version according to your minimum-stability setting
Installation failed, reverting ./composer.json to its original content.