Your requirements could not be resolved to an installable set of packages. Problem 1 - Conclusion: don't install vimeo/psalm 5.23.x-dev (conflict analysis result) - Conclusion: don't install vimeo/psalm 5.23.1 (conflict analysis result) - Root composer.json requires vimeo/psalm ^5.23.1 -> satisfiable by vimeo/psalm[5.23.1, 5.23.x-dev, 5.x-dev]. - Conclusion: don't install nikic/php-parser v4.16.0 (conflict analysis result) - vimeo/psalm 5.x-dev conflicts with nikic/php-parser v4.17.0. - Root composer.json requires infection/infection ^0.28.1 -> satisfiable by infection/infection[0.28.1, 0.28.x-dev]. - vimeo/psalm 5.x-dev requires nikic/php-parser ^4.16 -> satisfiable by nikic/php-parser[v4.16.0, ..., 4.x-dev]. - You can only install one version of a package, so only one of these can be installed: nikic/php-parser[v4.6.0, ..., 4.x-dev, v5.0.0alpha1, ..., 5.0.x-dev]. - infection/infection[0.28.1, ..., 0.28.x-dev] require nikic/php-parser ^5.0 -> satisfiable by nikic/php-parser[v5.0.0alpha1, ..., 5.0.x-dev]. - Conclusion: don't install nikic/php-parser v5.0.0 (conflict analysis result) Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.