composer/doc/DefaultPolicy.md

Default Solver Policy

A solver policy defines behaviour variables of the dependency solver. It decides which versions are considered newer than others, which packages should be prefered over others and whether operations like downgrades or uninstall are allowed.

Selection of prefered Packages

The following describe package pool situations with user requests and the resulting order in which the solver will try to install them.

The rules are to be applied in the order of these descriptions.

Package versions

Packages: Av1, Av2, Av3 Installed: Av2

Request: install A

(Av3)

Repository priorities

Packages Repo1.Av1, Repo2.Av1

priority(Repo1) >= priority(Repo2) => (Repo1.Av1, Repo2.Av1) priority(Repo2) < priority(Repo2) => (Repo2.Av1, Repo1.Av1)

Virtual Packages (provides)

Packages Av1, Bv1 Av1 provides Xv1 Bv1 provides Xv1

Request: install X

priority(Av1.repo) >= priority(Bv1.repo) => (Av1, Bv1) priority(Av1.repo) < priority(Bv1.repo) => (Bv1, Av1)

Package replacements

Packages: Av1, Bv2 Bv2 replaces Av1

Request: install A

priority(Av1.repo) > priority(Bv2.repo) => (Av1, Bv2) priority(Av1.repo) = priority(Bv2.repo) => (Av1, Bv2) priority(Av1.repo) < priority(Bv2.repo) => (Bv2, Av1)

Bv2.version is ignored, only the replacement version for A matters.