gistfile1.php

<?php

        $rootNode
            ->children()
                ->scalarNode('path')
                    ->defaultValue('%kernel.cache_dir%/gitprojects')
                    ->validate()->ifTrue(function($value){
                            return !file_exists($value); 
                         })->thenInvalid('Path must exist')
                        ->end()
                ->end()
            ->end();