Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires infection/infection ^0.29.6 -> satisfiable by infection/infection[0.29.6, 0.29.x-dev]. - infection/infection[0.29.6, ..., 0.29.x-dev] require nikic/php-parser ^5.0 -> found nikic/php-parser[dev-master, v5.0.0alpha1, ..., v5.1.0] but it conflicts with your root composer.json require (^4.19.1). Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.