Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires psalm/plugin-phpunit ^0.19.0 -> satisfiable by psalm/plugin-phpunit[0.19.0]. - infection/infection[0.29.0, ..., 0.29.6] require nikic/php-parser ^5.0 -> satisfiable by nikic/php-parser[v5.0.0, v5.0.1, v5.0.2, v5.1.0]. - Conclusion: don't install nikic/php-parser v5.1.0 (conflict analysis result) - Root composer.json requires infection/infection ^0.29.0 -> satisfiable by infection/infection[0.29.0, ..., 0.29.6]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.