- The new platform-check feature means that Composer checks the runtime PHP version and available extensions to ensure they match the project dependencies. If a mismatch is found, it exits with error details to make sure problems are not overlooked. To avoid issues when deploying to production it is recommended to run `composer check-platform-reqs` with the production PHP process as part of your build or deployment process.
- If a package exists in a higher priority repository, it will now be entirely ignored in lower priority repositories. See [repository priorities](https://getcomposer.org/repoprio) for details.
- Invalid PSR-0 / PSR-4 class configurations will not autoload anymore in optimized-autoloader mode, as per the warnings introduced in 1.10
- On linux systems supporting the XDG Base Directory Specification, Composer will now prefer using XDG_CONFIG_DIR/composer over ~/.composer if both are available (1.x used ~/.composer first)
- Package names now must comply to our [naming guidelines](doc/04-schema.md#name) or Composer will abort, as per the warnings introduced in 1.8.1
- Deprecated --no-suggest flag as it is not needed anymore
- PEAR support (repository, downloader, etc.) has been removed