Your requirements could not be resolved to an installable set of packages. Problem 1 - Conclusion: don't install vimeo/psalm 5.26.1 (conflict analysis result) - infection/infection[0.29.6, ..., 0.29.x-dev] require infection/mutator ^0.4 -> satisfiable by infection/mutator[0.4.0]. - Root composer.json requires infection/infection ^0.29.6 -> satisfiable by infection/infection[0.29.6, 0.29.x-dev]. - vimeo/psalm 5.x-dev conflicts with nikic/php-parser v4.17.0. - Root composer.json requires vimeo/psalm ^5.26.1 -> satisfiable by vimeo/psalm[5.26.1, 5.x-dev]. - vimeo/psalm 5.x-dev requires nikic/php-parser ^4.17 -> satisfiable by nikic/php-parser[v4.17.0, ..., 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, ..., v5.1.0]. - infection/mutator 0.4.0 requires nikic/php-parser ^5.0 -> satisfiable by nikic/php-parser[v5.0.0alpha1, ..., v5.1.0]. - Conclusion: don't install nikic/php-parser v5.1.0 (conflict analysis result) Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.