Your requirements could not be resolved to an installable set of packages. Problem 1 - Root composer.json requires infection/infection ^0.29 -> satisfiable by infection/infection[0.29.0, ..., 0.29.14]. - infection/infection[0.29.9, ..., 0.29.14] require nikic/php-parser ^5.3 -> found nikic/php-parser[v5.3.0, ..., v5.6.2] but it conflicts with your root composer.json require (^4). - infection/infection[0.29.0, ..., 0.29.8] require nikic/php-parser ^5.0 -> found nikic/php-parser[v5.0.0, ..., v5.6.2] but it conflicts with your root composer.json require (^4).