This is more here just in case any one else hits this error when trying to install Symfony.
PHP Fatal error: Class 'appDevDebugProjectContainer' not found in app/bootstrap.php.cache on line 2441(null) Script Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache handling the post-install-cmd event terminated with an exception
Its due to that warning you totally ignored when installing composer.
curl -sS https://getcomposer.org/installer | php #!/usr/bin/env php Some settings on your machine may cause stability issues with Composer. If you encounter issues, try to change the following: The apc.enable_cli setting is incorrect. Add the following to the end of your `php.ini`: apc.enable_cli = Off The php.ini used by your command-line PHP is: /etc/php5/cli/php.ini If you can not modify the ini file, you can also run `php -d option=value` to modify ini values on the fly. You can use -d multiple times. Downloading...
Make sure you set that
apc.enable_cli value. Then all will be well.