Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires infection/infection ^0.29.8 -> satisfiable by infection/infection[0.29.8]. - infection/mutator 0.4.0 requires nikic/php-parser ^5.0 -> satisfiable by nikic/php-parser[v5.0.0, ..., v5.3.1]. - You can only install one version of a package, so only one of these can be installed: nikic/php-parser[v4.17.0, ..., v4.19.4, v5.0.0, ..., v5.3.1]. - maglnet/composer-require-checker 4.14.0 requires nikic/php-parser ^4.19.4 -> satisfiable by nikic/php-parser[v4.19.4]. - infection/infection 0.29.8 requires infection/mutator ^0.4 -> satisfiable by infection/mutator[0.4.0]. - Root composer.json requires maglnet/composer-require-checker ^4.14.0 -> satisfiable by maglnet/composer-require-checker[4.14.0]. Use the option --with-all-dependencies (-W) to allow upgrades, downgrades and removals for packages currently locked to specific versions.