Choosing Your First Composer Version Constraint

When you publish a PHP library, Composer reads its composer.json file to learn which releases of other packages your code supports. A version constraint is the rule that defines those allowed releases. For a library dependency that follows Semantic Versioning, a release convention where major versions signal incompatible changes and minor and patch versions preserve backward compatibility, start with a caret constraint. Choose the earliest release your code supports, then use ^ to allow compatible updates within the same major version.

For example, ^3.4.1 allows releases from 3.4.1 up to, but not including, 4.0.0. That range makes a compatibility promise to people who install your library. Set its lower bound from the dependency features your code uses, and test the versions the range allows. The constraint describes a dependency requirement, not the version number you assign to your own package.

Set the lower bound from the APIs you use

The lower bound is the oldest dependency release your package claims to support. If your code calls a method introduced in version 3.4.1, a constraint beginning at 3.4.1 reflects that requirement. If the code also works with 3.0, beginning at 3.4.1 unnecessarily excludes users who could install your package.

Check the dependency’s release notes or documentation to find when the APIs your code relies on appeared. Then test against the oldest version you plan to allow. Composer recommends that library maintainers run composer update --prefer-lowest to check whether the declared minimum works. Also test against a current version permitted by your constraint. These checks cover the low end and a newer release, but they do not prove compatibility with every version in between. The Composer library guidance describes this testing approach.

Your requirement might look like this:

{
  "require": {
    "vendor/library": "^3.4.1"
  }
}

Use the actual dependency name and a lower bound supported by your code and tests. Avoid choosing the lower bound solely because it matches the version installed on your development machine.

Choose an upper bound that matches the dependency’s release policy

The caret operator permits compatible updates while stopping at the next major version for stable releases. In the example, ^3.4.1 means >=3.4.1 <4.0.0. Composer recommends this as the default for library dependencies that follow Semantic Versioning. See Composer’s constraint rules and the Semantic Versioning specification.

A constraint such as >=3.4.1 has no upper bound, so it also allows a future major release. If that release changes an API your library uses, Composer can select it and leave users with a broken installation. Once you publish a package release, you cannot change the requirement in that release; you must publish another version with a corrected constraint. Composer explains this risk in its guidance on unbounded constraints.

A tighter constraint can make sense when your package depends on behavior that only exists in a specific minor line. For example, ~3.4.1 allows releases from 3.4.1 up to, but not including, 3.5.0. That limits updates more than ^3.4.1, but also blocks later minor releases that might work. Use a narrower range when the dependency’s compatibility policy or your package’s actual requirements justify it, rather than as a substitute for testing.

Caret constraints also behave more conservatively before version 1.0.0. For example, ^0.3 allows versions from 0.3.0 up to, but not including, 0.4.0. That matters because Semantic Versioning treats 0.y.z releases as initial development, when the public interface is not considered stable.

Keep stability separate from compatibility

A version range answers which releases your package supports. Stability settings tell Composer whether it can choose prereleases, such as beta or development releases, when selecting dependency versions to install. Composer’s default minimum stability for a root project, the project whose composer.json runs Composer, is stable. A project that installs your library can make an exception for one dependency with a stability flag. The composer.json schema documents these settings.

For a normal library release, declare the compatible stable range and leave prereleases out of the requirement unless your package deliberately depends on one. A library generally cannot set the minimum-stability value that applies only to the project installing it. If a prerelease is required, make that choice explicit in the dependency requirement and explain why users need it.

Test the published promise and plan upgrades

A constraint states which dependency versions your package claims to support. Tests check whether your code works with some of those versions. In continuous integration, test the lowest permitted dependency versions and a current permitted set. Keep the library’s composer.lock file current for reproducible development and testing, but do not treat it as the consumer-facing compatibility rule. Composer uses the published constraints to choose dependency versions for projects that install your library. The library’s lock file does not freeze those projects’ installations. The library documentation explains that distinction.

When a dependency publishes a new major version, test your package against it before widening the constraint. If your code works, update the requirement and publish a new release. If the dependency does not follow its stated compatibility policy, narrow or adjust the range to reflect what you have verified.

Apply the same evidence-based approach to PHP and extension requirements. Composer treats PHP and extensions as platform packages, meaning they describe the environment rather than installable library code. Declare the oldest PHP version your package supports and test against it; Composer checks that requirement against the PHP version running Composer. See Composer’s platform dependency documentation.

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