Aliases

Why aliases?

When you are using a VCS repository, you will only get comparable versions for branches that look like versions, such as 2.0. For your master branch, you will get a dev-master version. For your bugfix branch, you will get a dev-bugfix version.

If your master branch is used to tag releases of the 1.0 development line, i.e. 1.0.1, 1.0.2, 1.0.3, etc., any package depending on it will probably require version 1.0.*.

If anyone wants to require the latest dev-master, they have a problem: Other packages may require 1.0.*, so requiring that dev version will lead to conflicts, since dev-master does not match the 1.0.* constraint.

Enter aliases.

Branch alias

The dev-master branch is one in your main VCS repo. It is rather common that someone will want the latest master dev version. Thus, Composer allows you to alias your dev-master branch to a 1.0.x-dev version. It is done by specifying a branch-alias field under extra in composer.json:

{
    "extra": {
        "branch-alias": {
            "dev-master": "1.0.x-dev"
        }
    }
}

The branch version must begin with dev- (non-comparable version), the alias must be a comparable dev version (i.e. start with numbers, and end with .x-dev). The branch-alias must be present on the branch that it references. For dev-master, you need to commit it on the master branch.

As a result, anyone can now require 1.0.* and it will happily install dev-master.

In order to use branch aliasing, you must own the repository of the package being aliased. If you want to alias a third party package without maintaining a fork of it, use inline aliases as described below.

Require inline alias

Branch aliases are great for aliasing main development lines. But in order to use them you need to have control over the source repository, and you need to commit changes to version control.

This is not really fun when you just want to try a bugfix of some library that is a dependency of your local project.

For this reason, you can alias packages in your require and require-dev fields. Let's say you found a bug in the monolog/monolog package. You cloned Monolog on GitHub and fixed the issue in a branch named bugfix. Now you want to install that version of monolog in your local project.

You are using symfony/monolog-bundle which requires monolog/monolog version 1.*. So you need your dev-bugfix to match that constraint.
Just add this to your project's root composer.json:

{
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/you/monolog"
        }
    ],
    "require": {
        "symfony/monolog-bundle": "2.0",
        "monolog/monolog": "dev-bugfix as 1.0.x-dev"
    }
}

That will fetch the dev-bugfix version of monolog/monolog from your GitHub and alias it to 1.0.x-dev.

Note: If a package with inline aliases is required, the alias (right of the as) is used as the version constraint. The part left of the as is discarded. As a consequence, if A requires B and B requires monolog/monolog version dev-bugfix as 1.0.x-dev, installing A will make B require 1.0.x-dev, which may exist as a branch alias or an actual 1.0 branch. If it does not, it must be re-inline-aliased in A's composer.json.

Note: Inline aliasing should be avoided, especially for published packages. If you found a bug, try and get your fix merged upstream. This helps to avoid issues for users of your package.

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