Symfony2, composer, your PHP version (5.6.18) overriden by “config.platform.php” version (5.3.9) does not satisfy requirement

Somewhere in your composer.json you have

"config": {
   "preferred-install": "dist",
   "platform": {
       "php": "5.3.9"
   }
}

That block overrides your current PHP version as described in composer doc.